Not adding user_id data to forced builds if it is not available

[campos-ddc]

This can happen when making force requests through an external tool, in these situations we do not have a session to extract user data from.

[campos-ddc]

This should just hide the problem, I don't know what happens if we don't have a user_id

[coveralls]

Coverage decreased (-0.001%) to 68.672% when pulling 23ae417 on ignore-when-user-not-available into d1888bd on staging.

[warcholprzemo]

We are prepared for this situation. Look a the forcesched.py: https://github.com/Unity-Technologies/katana/pull/358/files#diff-dc9b93454efa012931c16afab1083f56
Later if user_id is None code try to find user_id in "old way". If it doesn't find the user_id, a row in the build_user is not added.