Fix pandas codec nan conversion for non-numeric columns

[guillermo-alicea]

Description

This changes fixes a bug in the PandasCodec to_response_output conversion function which raises the following error when null values are attempted to be checked as nan values using numpy.isnan(x). This can occur in cases such as a string series with nullable entries.

TypeError: ufunc 'isnan' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''

Changes Made

• Update PandasCodec ResponseOutput mapper to skip nan conversion for BYTES column
  • This change mirrors the behavior present in NumpyCodec
• Update PandasCodec string content type detection to ignore null values in an otherwise string column

Related Issues

• https://github.com/SeldonIO/MLServer/issues/1873
• https://github.com/SeldonIO/MLServer/issues/1747#issuecomment-2411056443

Screenshots (if applicable)

N/A

Checklist

• [x] Code follows the project's style guidelines
• [x] All tests related to the changes pass successfully
• [x] Documentation is updated (if necessary)
• [ ] Code is reviewed by at least one other team member
• [ ] Any breaking changes are communicated and documented

Additional Notes

N/A

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