Fix minor C type confusion

Returning false instead of NULL gets a compiler error under gcc-14
-std=gnu23, and it appears to have been unintentional.  Fix for commit
8492feb98f.

src/backend/utils/sort/tuplesortvariants.c

@@ -1099,7 +1099,7 @@ tuplesort_getgintuple(Tuplesortstate *state, Size *len, bool forward)
 	MemoryContextSwitchTo(oldcontext);
 
 	if (!stup.tuple)
-		return false;
+		return NULL;
 
 	tup = (GinTuple *) stup.tuple;