Remove check for NULL in STRICT function

test_bms_make_singleton is defined as STRICT and only takes a single
parameter, so there is no need to check that parameter for NULL as a
NULL input won't ever reach there.

Author: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/BC483901-9587-4076-B20F-9A414C66AB78@yesql.se

src/test/modules/test_bitmapset/test_bitmapset.c

@@ -201,9 +201,6 @@ test_bms_make_singleton(PG_FUNCTION_ARGS)
 	Bitmapset  *bms;
 	int32		member;
 
-	if (PG_ARGISNULL(0))
-		PG_RETURN_NULL();		/* invalid input */
-
 	member = PG_GETARG_INT32(0);
 	bms = bms_make_singleton(member);