[SPARK-50491][SQL] Fix bug where empty BEGIN END blocks throw an error

[dusantism-db]

This PR depends on #48989

What changes were proposed in this pull request?

There is a bug in SQL scripting which causes empty compound statements to throw an error if their body consists solely of empty BEGIN END blocks. Examples:

WHILE 1 = 1 DO
  BEGIN
  END;
END WHILE;

BEGIN
  BEGIN
    BEGIN
    END;
  END;
END;

This PR fixes this by introducing a NO-OP statement for SQL scripting, which empty BEGIN END blocks will return.

Why are the changes needed?

Currenty, compound bodies declare they have the next element even if their body is consisted only of empty blocks. This is because it only checks for existence of statements in the body, not whether there is at least one statement which is not an empty block.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Unit tests were added to existing suites.

Was this patch authored or co-authored using generative AI tooling?

No.

[miland-db]

Good job on testing different NonLeafStatements with empty compounds!

[cloud-fan]

This approach LGTM, but do you know why CompoundBodyExec can't handle Nil statements?

[dusantism-db]

This approach LGTM, but do you know why CompoundBodyExec can't handle Nil statements?

@cloud-fan
The problem is that hasNext for compound bodies is true for non-empty bodies, however a non-empty body with only empty BEGIN END blocks does not have a leaf statement to return. So Compound body expects a leaf statement because hasNext is true, but since there is no actual statement we will return the NO-OP.

[cloud-fan]

thanks, merging to master!