Remove batch_source required field for PushSource #5440

Is your feature request related to a problem? Please describe.
I have a PushSource which I used at offline time to push features that I created during training (for example, embedding of entity after model training),

The problem is that PushSource requires me to provide a batch_source but I dont have this kind of features before training (Because this features are generated only after training) so I must do this overhead/workaround:

• Create a dummy parquet file to store at least one value (else if I remember right I got an error)
• Create a FileSource to point the parquet file.
• Point the batch_source to the FileSource
  All of this need to done for each feature view of a push source that I want,

Describe the solution you'd like
Expected behavour will be to leave the batch_source empty and only create FV and PushSource

Additional context
You can see example of the workaround here