Initialize Movie.trakt property with None

[glensc]

Various calls to extract_ids create the trakt property. Add the initializing to IDE typing would not consider property unknown.

For example:

• https://github.com/Taxel/PlexTraktSync/blob/88dc3fd97e603f73316d5bd393bce9f7447aeb94/plextraktsync/sync.py#L119

Relates to some older bug report:

• AttributeError: 'Movie' object has no attribute 'trakt' moogar0880/PyTrakt#158

[glensc]

Perhaps should finish this one:

• [python-pytrakt] Refactor: Add IdsMixin to replace extract_ids() utility method moogar0880/PyTrakt#186