Prevent a crash if MusicBrainz release date is missing

[ribbons]

If MusicBrainz returns data for a CD being ripped but no release date is returned (e.g. https://musicbrainz.org/release/340b94f5-139e-4f7b-90dd-82d04a70c57b), we attempt to set the DATE tag to None (which is caught by mutagen), causing the following crash:

Traceback (most recent call last):                
  File "/usr/local/lib/python2.7/dist-packages/whipper-0.4.2-py2.7.egg/morituri/extern/task/task.py", line 511, in c
    callable(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/whipper-0.4.2-py2.7.egg/morituri/common/encode.py", line 227, in _tag
    w.save()
  File "/usr/local/lib/python2.7/dist-packages/mutagen/_util.py", line 158, in wrapper
    return func(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/mutagen/_util.py", line 129, in wrapper
    return func(self, h, *args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/mutagen/flac.py", line 860, in save
    self.metadata_blocks, available, content_size, padding)
  File "/usr/local/lib/python2.7/dist-packages/mutagen/flac.py", line 154, in _writeblocks
    data += cls._writeblock(block)
  File "/usr/local/lib/python2.7/dist-packages/mutagen/flac.py", line 126, in _writeblock
    datum = block.write()
  File "/usr/local/lib/python2.7/dist-packages/mutagen/flac.py", line 354, in write
    return super(VCFLACDict, self).write(framing=framing)
  File "/usr/local/lib/python2.7/dist-packages/mutagen/_vorbis.py", line 191, in write
    self.validate()
  File "/usr/local/lib/python2.7/dist-packages/mutagen/_vorbis.py", line 171, in validate
    raise ValueError("%r is not a valid value" % value)
ValueError: None is not a valid value
CRITICAL:morituri.command.cd:Giving up on track 1 after 5 times
Traceback (most recent call last):
  File "/usr/local/bin/whipper", line 11, in <module>
    load_entry_point('whipper==0.4.2', 'console_scripts', 'whipper')()
  File "/usr/local/lib/python2.7/dist-packages/whipper-0.4.2-py2.7.egg/morituri/command/main.py", line 31, in main
    ret = cmd.do()
  File "/usr/local/lib/python2.7/dist-packages/whipper-0.4.2-py2.7.egg/morituri/command/basecommand.py", line 123, in do
    return self.cmd.do()
  File "/usr/local/lib/python2.7/dist-packages/whipper-0.4.2-py2.7.egg/morituri/command/basecommand.py", line 123, in do
    return self.cmd.do()
  File "/usr/local/lib/python2.7/dist-packages/whipper-0.4.2-py2.7.egg/morituri/command/cd.py", line 206, in do
    self.doCommand()
  File "/usr/local/lib/python2.7/dist-packages/whipper-0.4.2-py2.7.egg/morituri/command/cd.py", line 507, in doCommand
    ripIfNotRipped(i + 1)
  File "/usr/local/lib/python2.7/dist-packages/whipper-0.4.2-py2.7.egg/morituri/command/cd.py", line 447, in ripIfNotRipped
    "track can't be ripped. "

To resolve the issue, this PR adds a check to see if there is a value for release date before setting it. I've not added a warning message, as this is already output before the crash occurs:

WARNING:morituri.common.mbngs:Release with ID '340b94f5-139e-4f7b-90dd-82d04a70c57b' (The Goons - 'Enter Bluebottle...') does not have a date

Not sure if this should have a unit test & if so where it should go (don't seem to be any MusicBrainz unit tests for track ripping?), so I've not added one - let me know if I should.

Fixes #133.

[JoeLametta]

Not sure if this should have a unit test & if so where it should go (don't seem to be any MusicBrainz unit tests for track ripping?), so I've not added one - let me know if I should.

I think that's fine (similar to what I've mentioned here in the bug report).

Thanks for the PR.

[ribbons]

I think that's fine (similar to what I've mentioned here in the bug report).

Thanks for the PR.

No worries - sorry, totally forgot to search for relevant bug reports :-/.