[mlir] Add check for value that might be null.

Because we are generating uninitialized value for no integer type and use `isUninitialized()` to judge if it is valid after https://reviews.llvm.org/rG93f081c896536112e1ca8133991d23cb1134793a, we should check the value before use `getValue` to get it.
Fixes https://github.com/llvm/llvm-project/issues/59984.

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D141661