Make source optional for FeatureView

Is your feature request related to a problem? Please describe.
Sometimes feature views do not have an offline source. They are written initially online and are sync'd offline by an external process so they cannot be associated with a push source.

Describe the solution you'd like
A feature view without a source can describe these feature views that exist only online.

Describe alternatives you've considered
We have explored ODFVs without a transformation but that solution was rejected.