VCS: check for the class before using it

[sentry-io[bot]]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: readthedocs/projects/models.py

Function	Unhandled Issue
get_default_branch	KeyError: 'version_identifier' /projects/{project... Event Count: 80
get_default_branch	KeyError: 'version_identifier' /_/api/v2/footer_h... Event Count: 48
get_default_branch	AttributeError: 'NoneType' object has no attribute 'fallback_branch' /_... Event Count: 36
get_default_branch	AttributeError: 'NoneType' object has no attribute 'fallback_branch' /proj... Event Count: 17

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