fix: handle schema retrieval for datafusion api

[xushiyan]

Description

Fix DataFusion integration get schema API to handle empty table case - the original Hudi table api returns an Err when retrieving schema fails. The DataFusion API expects a legit SchemaRef to return, so change to return empty schema when Hudi table api gives Err.

Resolves #126

How are the changes test-covered

• N/A
• Automated tests (unit and/or integration tests)
• Manual tests
  • Details are described below

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.93%. Comparing base (bf5c6dd) to head (759c01a).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #187      +/-   ##
==========================================
+ Coverage   90.81%   90.93%   +0.12%     
==========================================
  Files          20       20              
  Lines         958      960       +2     
==========================================
+ Hits          870      873       +3     
+ Misses         88       87       -1     

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