Fix null not working with comparisons.

sourcepawn/compiler/tests/ok-inheritance-null.sp

@@ -0,0 +1,19 @@
+enum Handle:
+{
+}
+
+methodmap Handle __nullable__
+{
+}
+
+methodmap StringMap < Handle
+{
+}
+
+native Log(const char[] fmt, any:...)
+
+public main()
+{
+	StringMap f
+	Log("hello %d", f != null)
+}