Fix 500 error on webserver after upgrading to FAB provider 1.5.4

[vatsrahul1001]

Fix 500 error on webserver after upgrading to FAB provider 1.5.4

FAB 1.5.4 bumped flask-session to >=0.8.0, which changed the session
serialization format (MsgPack to JSON). After upgrading, browsers with
an existing session cookie from 1.5.3 cause AirflowTaggedJSONSerializer.decode()
to blow up, which propagates out of open_session() leaving ctx.session as
None.

We should create a new session rather than letting the error propagate.

• Yes (please specify the tool below)
  Claude

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