Restore 0-argument love.graphics.newFont/love.font.newRasterizer

Fix subtle bug in luax_convobj in the process, it couldn't deal with 0 arguments
before.

src/common/runtime.cpp

@@ -489,7 +489,8 @@ int luax_convobj(lua_State *L, int idxs[], int n, const char *mod, const char *f
 	lua_call(L, n, 2); // Call the function, n args, one return value (plus optional errstring.)
 	luax_assert_nilerror(L, -2); // Make sure the function returned something.
 	lua_pop(L, 1); // Pop the second return value now that we don't need it.
-	lua_replace(L, idxs[0]); // Replace the initial argument with the new object.
+	if (n > 0)
+		lua_replace(L, idxs[0]); // Replace the initial argument with the new object.
 	return 0;
 }