Python ULR Harness submits null pipeline_options field in PrepareJobRequest

the beam_job_api gRPC definition indicates that pipeline_options is a required field.  However, the python ULR currently does not set an options struct.  This is trivially solved on the job server side by instantiating a default.  However, there may be cases where the job server assumes it exists and falls over on an NPE during translation.

Imported from Jira [BEAM-3805](https://issues.apache.org/jira/browse/BEAM-3805). Original Jira may contain additional context.
Reported by: axelmagn.

Start by reading the beam_job_api gRPC definition and the Python ULR code that constructs PrepareJobRequest. Trace how pipeline_options is populated, then verify that the request always carries an options struct and that translation no longer receives a null value.

Written by the indexing model from the issue text.