Coverage fails to generate report with 0% coverage

[wlcx]

Describe the bug
Coverage >5.0a6 seems to fail to generate a report when coverage is 0%, when run by pytest-cov under tox.

To Reproduce
Please see https://github.com/wlcx/coverage-bug-repro for a minimum working reproduction of this. Essentially, it is a single module project, with a test that doesn't actually result in any code coverage. I appreciate this is a bit of a useless edge case, but this did used to not cause any errors!

• Run tox (which runs python -m pytest --cov=foo tests/):

PS C:\Users\sam.willcocks\prog\coverage-bug-repro> tox
py3 recreate: C:\Users\sam.willcocks\prog\coverage-bug-repro\.tox\py3
py3 installdeps: py, pytest, pytest-cov, coverage
py3 installed: atomicwrites==1.3.0,attrs==19.3.0,colorama==0.4.3,coverage==5.0,importlib-metadata==1.3.0,more-itertools==8.0.2,packaging==19.2,pluggy==0.13.1,py==1.8.0,pyparsing==2.4.5,pytest==5.3.2,pytest-cov==2.8.1,six==1.13.0,wcwidth==0.1.7,zipp==0.6.0
py3 run-test-pre: PYTHONHASHSEED='286'
py3 run-test: commands[0] | python -m pytest --cov=foo tests/
=========================================================== test session starts ============================================================ 
platform win32 -- Python 3.7.4, pytest-5.3.2, py-1.8.0, pluggy-0.13.1
cachedir: .tox\py3\.pytest_cache
rootdir: C:\Users\sam.willcocks\prog\coverage-bug-repro
plugins: cov-2.8.1
collected 1 item                                                                                                                             

tests\test_nothing.py .                                                                                                               [100%]Coverage.py warning: No data was collected. (no-data-collected)
WARNING: Failed to generate report: No data to report.

C:\Users\sam.willcocks\prog\coverage-bug-repro\.tox\py3\lib\site-packages\pytest_cov\plugin.py:254: PytestWarning: Failed to generate report: No data to report.

  self.cov_controller.finish()


----------- coverage: platform win32, python 3.7.4-final-0 -----------


============================================================ 1 passed in 0.07s ============================================================= 
_________________________________________________________________ summary __________________________________________________________________ 
  py3: commands succeeded
  congratulations :)

• then, subsequently running tox -e coverage-report (running coverage html):

PS C:\Users\sam.willcocks\prog\coverage-bug-repro> tox -e coverage-report   
coverage-report recreate: C:\Users\sam.willcocks\prog\coverage-bug-repro\.tox\coverage-report
coverage-report installdeps: coverage
coverage-report installed: coverage==5.0
coverage-report run-test-pre: PYTHONHASHSEED='242'
coverage-report run-test: commands[0] | coverage html
No data to report.
ERROR: InvocationError for command 'C:\Users\sam.willcocks\prog\coverage-bug-repro\.tox\coverage-report\Scripts\coverage.EXE' html (exited with code 1)
_________________________________________________________________ summary __________________________________________________________________ 
ERROR:   coverage-report: commands failed

Expected behavior
If I pin the version of coverage used to ==5.0a5:

PS C:\Users\sam.willcocks\prog\coverage-bug-repro> tox
py3 recreate: C:\Users\sam.willcocks\prog\coverage-bug-repro\.tox\py3
py3 installdeps: py, pytest, pytest-cov, coverage==5.0a5
py3 installed: atomicwrites==1.3.0,attrs==19.3.0,colorama==0.4.3,coverage==5.0a5,importlib-metadata==1.3.0,more-itertools==8.0.2,packaging==19.2,pluggy==0.13.1,py==1.8.0,pyparsing==2.4.5,pytest==5.3.2,pytest-cov==2.8.1,six==1.13.0,wcwidth==0.1.7,zipp==0.6.0
py3 run-test-pre: PYTHONHASHSEED='845'
py3 run-test: commands[0] | python -m pytest --cov=foo tests/
=========================================================== test session starts ============================================================ 
platform win32 -- Python 3.7.4, pytest-5.3.2, py-1.8.0, pluggy-0.13.1
cachedir: .tox\py3\.pytest_cache
rootdir: C:\Users\sam.willcocks\prog\coverage-bug-repro
plugins: cov-2.8.1
collected 1 item                                                                                                                             

tests\test_nothing.py .                                                                                                               [100%]Coverage.py warning: No data was collected. (no-data-collected)


----------- coverage: platform win32, python 3.7.4-final-0 -----------
Name         Stmts   Miss  Cover
--------------------------------
foo\bar.py       2      2     0%


============================================================ 1 passed in 0.21s ============================================================= 
_________________________________________________________________ summary __________________________________________________________________ 
  py3: commands succeeded
  congratulations :)

• then, running coverage html:

PS C:\Users\sam.willcocks\prog\coverage-bug-repro> tox -e coverage-report   
coverage-report recreate: C:\Users\sam.willcocks\prog\coverage-bug-repro\.tox\coverage-report
coverage-report installdeps: coverage==5.0a5
coverage-report installed: coverage==5.0a5
coverage-report run-test-pre: PYTHONHASHSEED='595'
coverage-report run-test: commands[0] | coverage html
_________________________________________________________________ summary __________________________________________________________________ 
  coverage-report: commands succeeded
  congratulations :)

[nedbat]

This is fixed in aefe08b

[nedbat]

This is now available in coverage 5.0.2