[mlir] don't drop undef initializers in translation to LLVM IR

LLVM IR allows globals with external linkage to have initializers, including
undef. The translation was incorrectly using undef as a indicator that the
initializer should be ignored in translation, leading to the impossibility to
create an external global with an explicit undef initializer. Fix this and use
nullptr as a marker instead.

Reviewed By: wsmoses

Differential Revision: https://reviews.llvm.org/D105631