[Moore][NFC] Fix warning when building Moore components

[cepheus69]

The original building warning:

[814/1036] Building CXX object lib/Dialect/Moore/CMakeFiles/obj.CIRCTMoore.dir/MooreOps.cpp.o
In file included from /home/ursa/Projects/circt/lib/Dialect/Moore/MooreOps.cpp:13:
In file included from /home/ursa/Projects/circt/include/circt/Dialect/Moore/MooreOps.h:16:
In file included from /home/ursa/Projects/circt/include/circt/Dialect/HW/HWTypes.h:17:
In file included from /home/ursa/Projects/circt/include/circt/Dialect/HW/HWDialect.h:16:
In file included from /home/ursa/Projects/circt/include/circt/Support/LLVM.h:22:
In file included from /home/ursa/Projects/circt/llvm/mlir/include/mlir/Support/LLVM.h:23:
/home/ursa/Projects/circt/llvm/llvm/include/llvm/Support/Casting.h:490:69: warning: returning reference to local temporary object [-Wreturn-stack-address]
  490 |   static inline CastReturnType castFailed() { return CastReturnType(nullptr); }
      |                                                                     ^~~~~~~
/home/ursa/Projects/circt/llvm/llvm/include/llvm/Support/Casting.h:494:14: note: in instantiation of member function 'llvm::CastInfo<circt::moore::SVModuleOp, const circt::moore::SVModuleOp>::castFailed' requested here
  494 |       return castFailed();
      |              ^
/home/ursa/Projects/circt/llvm/llvm/include/llvm/Support/Casting.h:651:36: note: in instantiation of member function 'llvm::CastInfo<circt::moore::SVModuleOp, const circt::moore::SVModuleOp>::doCastIfPossible' requested here
  651 |   return CastInfo<To, const From>::doCastIfPossible(Val);
      |                                    ^
1 warning generated.

The main problem is handling the result coming from the cast function. To check if the pointer pointing to a module op is a nullptr. This commit rewrites the appropriate code to fix the warning.

[hailongSun2000]

LGTM!