[mlir][LLVMIR] Support switch without cases

LLVM supports switch ops without case values, for which the optional
caseValues has no value (i.e., std::nullopt, no the empty array).
Handle this case properly when translating MLIR-LLVM to LLVM-IR.

Reviewed By: Dinistro, Mogball

Differential Revision: https://reviews.llvm.org/D156431