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
• [x] Automated tests (unit and/or integration tests)
• [ ] Manual tests
  • [ ] Details are described below

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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