fix: Table chart dimension fallback in normalize_dimensions

[sentry]

This PR addresses the ValueError: At least one dimension is required for table charts (DALGO-BACKEND-27X) occurring when requesting data previews for table charts via the public API.

Problem:
Table charts were failing if dimensions were specified using the legacy dimension_col and extra_dimension fields, while the dimensions list in the payload was empty. The normalize_dimensions function, specifically for chart_type == "table", was only checking payload.dimensions and not falling back to the legacy fields, leading to an empty list of dimensions being returned.

Root Cause:
The normalize_dimensions function had a missing fallback mechanism for table charts. While the non-table chart branch correctly utilized dimension_col and extra_dimension when payload.dimensions was not provided, the table chart branch did not, despite its docstring indicating backward compatibility handling.

Solution:
Implemented a fallback in normalize_dimensions for table charts. If payload.dimensions is empty or contains only blank strings, the function now populates the dimensions list using payload.dimension_col and payload.extra_dimension, ensuring that charts configured with these legacy fields function correctly.

Fixes DALGO-BACKEND-27X