pgsql: Provide stable test for NULL-values in KNN SP-GiST

Provide stable test for NULL-values in KNN SP-GiST f5f084fc3e has removed test because of its instability. This commit provides alternative test with determined ordering using extra ORDER BY expression. Backpatch-through: 12 Branch -- REL_12_STABLE Details ---

pgsql: Provide stable test for NULL-values in KNN SP-GiST

Provide stable test for NULL-values in KNN SP-GiST f5f084fc3e has removed test because of its instability. This commit provides alternative test with determined ordering using extra ORDER BY expression. Backpatch-through: 12 Branch -- master Details ---