GH-3307: Replace deprecated BIT_PACKED with RLE in DevNullValuesWriter

[LuciferYang]

Rationale for this change

Fixes #3307

When repetition/definition levels are empty (i.e., maxLevel == 0 for required, non-repeated fields), DevNullValuesWriter is used as a no-op writer that produces zero bytes. However, its getEncoding() method returns the deprecated BIT_PACKED encoding, which gets written into the DataPageHeader metadata (repetition_level_encoding / definition_level_encoding) and the column chunk encoding list.

Since parquet-java already uses RLE as the encoding for levels, the metadata should reflect RLE rather than the deprecated BIT_PACKED.

What changes are included in this PR?

Changed DevNullValuesWriter.getEncoding() to return Encoding.RLE instead of Encoding.BIT_PACKED.

Are these changes tested?

Yes. All existing tests in parquet-column and parquet-hadoop pass without modification.

Are there any user-facing changes?

Newly written Parquet files will report RLE instead of BIT_PACKED in page header metadata for empty repetition/definition levels. This has no impact on file compatibility — readers do not decode level data when the byte length is zero, regardless of the encoding value in the header.