API: Handle null partition values in ResidualEvaluator startsWith/notStartsWith

[raghavyadav01]

Closes #18412

ResidualEvaluator dereferences the partition value in startsWith and notStartsWith without a null check, so computing a residual for a partition whose value is null throws NullPointerException:

java.lang.NullPointerException: Cannot invoke "String.startsWith(String)" because the return value of
"org.apache.iceberg.expressions.BoundReference.eval(org.apache.iceberg.StructLike)" is null
	at org.apache.iceberg.expressions.ResidualEvaluator$ResidualVisitor.startsWith(ResidualEvaluator.java:215)

ManifestFilterManager computes a residual for every live file in a manifest it rewrites, so a delete by row filter fails whenever the manifest also holds a file with a null partition value. Scans hit it too: with or(isNull(s), startsWith(s, "a")) the null-partition file survives planning through the isNull branch, and FileScanTask.residual() then throws.

Change

Treat a null partition value as not matching, so startsWith returns alwaysFalse() and notStartsWith returns alwaysTrue().

This keeps ResidualEvaluator consistent with Evaluator, which already evaluates startsWith as evalRes != null && ((String) evalRes).startsWith(...) and notStartsWith as its negation, and it mirrors the guard in and notIn received in #17014.

Tests

Added testStartsWithWithNullStringPartition and testNotStartsWithWithNullStringPartition to TestResiduals, following the shape of the existing testInWithNullStringPartition / testNotInWithNullStringPartition cases. Both fail with NullPointerException on main and pass with this change.