Handle null TableCell entries in table/data_table block rows

[VictorHLi404]

Summary

• Adds NullSafeTableCellRowDeserializer and NullSafeDataTableCellRowDeserializer — custom Jackson deserializers that substitute a blank RawTextTableCell for any null cell entries within a row
• Annotates getRows() in both TableIF and DataTableIF with @JsonDeserialize(contentUsing = ...) to wire in the deserializers
• Adds test fixtures and assertions for null-cell deserialization in both block types

NOTE: this PR is to address null problems observed in production. Slack is returning table/data_table blocks where individual cells within a row are null, causing deserialization to fail with: "Guava Collection of type ImmutableList does not accept null values". Using blank RawTextTableCell (empty string) rather than skipping nulls preserves row structure so DataTableIF's column-count validation continues to work correctly.

Test plan

• TableBlockTest#itDeserializesNullCellsAsBlankRawTextCells — null cell in a table row becomes RawTextTableCell.of("")
• DataTableBlockTest#itDeserializesNullCellsAsBlankRawTextCells — null cells in both header and body rows are substituted correctly
• All existing table and data_table block tests continue to pass (271 tests green locally)