[UR] Check null pointer before handle in validation layer

[Bensuo]

• Adjust mako template for ur_valddi.cpp so that pointers are null-checked before handles
• Fixes issue with handles inside a pointer to a struct where the pointer would be dereferenced to check for a null handle before the pointer itself was null checked.
• Updates some tests which passed additional null pointer args when checking for null handle errors, which would fail since the null pointer errors take priority now.

Issue was found during coverity scan.

[Bensuo]

@intel/llvm-gatekeepers This should be good to merge now, thanks!