[mlir] Add an assertion on creating an Operation with null result types

Null types are commonly used as an error marker. Catch them in the constructor
of Operation if they are present in the result type list, as otherwise this
could lead to further surprising behavior when querying op result types.

Fix AsyncToLLVM and StandardToLLVM that were using null types when constructing
operations.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D91770