Remove unnecessary null check.

This checked that individual char arrays in the MJCF schema were not null, which would never really happen, and triggered some compiler warnings

PiperOrigin-RevId: 520348916
Change-Id: Ic8b82737460ed3bcf44d9629cd0c7de5782bee8c

src/xml/xml_util.cc

@@ -133,7 +133,7 @@ mjXSchema::mjXSchema(const char* schema[][mjXATTRNUM], int nrow, bool checkptr)
 
     for (int i=0; i<nrow; i++) {
       // base pointers
-      if (!schema[i] || !schema[i][0]) {
+      if (!schema[i][0]) {
         mju::sprintf_arr(msg, "null pointer found in row %d", i);
         error = msg;
         return;