[flink] Fix scan partitions with partial partition spec

[ArnavBalyan]

Purpose

• scan.partitions incorrectly returns data of all partitions, when partial partition spec is provided in the query.
• The loader passes the incomplete specification to the partition converter. This leads to an exception which is swallowed and the partition filter is completely discarded as a result.
• Once the partition filter is discarded, the original partial spec is not honored and all rows are returned instead of the partition values.
• Ensure that missing partial keys are filled with partition.default-name to safely create the partition spec.

Tests

• UT