[OCaml] Handle nullptr in Llvm.global_initializer

LLVMGetInitializer returns nullptr in case there is no initializer.
There is not much that can be done with nullptr in OCaml, not even
test if it is null. Also, there does not seem to be a C or OCaml API
to test if there is an initializer. So this diff changes
Llvm.global_initializer to return an option.

Reviewed By: whitequark

Differential Revision: https://reviews.llvm.org/D65195