Fix Painless void return bug (#38046)

Painless now allows void functions and contexts to with a void return type to use
a return statement without a following expression.

Author: jdconrad

modules/lang-painless/src/main/antlr/PainlessParser.g4

@@ -55,7 +55,7 @@ dstatement
     | declaration                     # decl
     | CONTINUE                        # continue
     | BREAK                           # break
-    | RETURN expression               # return
+    | RETURN expression?              # return
     | THROW expression                # throw
     | expression                      # expr
     ;