Removing empty argument list fallback breaks objective-c test.

[emilio]

CC @simlay

In #1781 I added a fallback path to the usual "grab the argument names" code:

rust-bindgen/src/ir/function.rs

Line 430 in b1a1ebc

args_from_ty_and_cursor(&ty, &cursor, ctx)

I found a better way to handle the original test-case (#1785), but the objective-c test that #1781 fixed is still not fixed by that.

I took a look at the clang ast (with --emit-clang-ast) and the arguments don't seem to be exposed there, so this probably needs libclang changes.