PR author can be null

[JuliaTagBot]

Repo: SciML/DiffEqOperators.jl
Run URL: https://github.com/SciML/DiffEqOperators.jl/actions/runs/185274288
Image ID: sha256:b92afa4e6e356bb0d110ec1fd2d43fad79121184478f14419d7a7d87ce4d014a
Stacktrace:

Traceback (most recent call last):
  File "/root/tagbot/action/__main__.py", line 84, in <module>
    repo.create_release(version, sha)
  File "/root/tagbot/action/repo.py", line 389, in create_release
    log = self._changelog.get(version, sha)
  File "/root/tagbot/action/changelog.py", line 215, in get
    data = self._collect_data(version, sha)
  File "/root/tagbot/action/changelog.py", line 199, in _collect_data
    "issues": [self._format_issue(i) for i in issues],
  File "/root/tagbot/action/changelog.py", line 199, in <listcomp>
    "issues": [self._format_issue(i) for i in issues],
  File "/root/tagbot/action/changelog.py", line 159, in _format_issue
    "closer": self._format_user(issue.closed_by),
  File "/root/tagbot/action/changelog.py", line 149, in _format_user
    "name": user.name or user.login,
AttributeError: 'NoneType' object has no attribute 'name'