update skip list for 8.12 (#810) (#812)

Co-authored-by: Laurent Saint-Félix <laurent.saintfelix@elastic.co>

Author: github-actions[bot]

internal/build/cmd/generate/commands/gentests/skips.go

@@ -501,6 +501,9 @@ esql/10_basic.yml:
 
 esql/20_aggs.yml:
 
+esql/25_aggs_on_null.yml:
+  - group on null, long
+
 esql/30_types.yml:
   - unsigned_long