Allow Dialects to be initialized via nullptr.

This allows Dialect to follow the MLIR style of nullable objects, and in fact is expected by `Dialect::operator bool() const` which already tests whether `def == nullptr`. This just wasn't a reachable situation, because the constructor was dereferencing the pointer unconditionally.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D86807