Refactor empty filter handling in GribSplitterV2

[j9sh264]

fix(splitter): gracefully handle empty GRIB filter matches

When applying a --where filter expression in GribSplitterV2, files containing
zero matching messages previously caused a crash or required a broad try/except
block in the pipeline that swallowed unexpected errors.

This change:

• Detects when grib_get returns an empty string (no filter matches).
• Gracefully logs a warning and returns early instead of crashing.
• Increments a new skipped_empty_filter Beam metric for observability.

[mahrsee1997]

LGTM!
Please bump version of setup.py and Version.txt.