[lld-macho] Sibling N_SO symbols must have the empty string

We had been giving them a string index of zero, which actually corresponds to a
string with a single space due to {D89639}.

This was far from obvious in the old test because llvm-nm doesn't quote the
symbol names, making the empty string look identical to a string of a single
space. `dsymutil -s` quotes its strings, so I've changed the test accordingly.

Fixes llvm.org/PR48714. Thanks @clayborg for the tips!

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D100003