Avoid extra work in global_symbol_searcher::expand_symtabs

I noticed that global_symbol_searcher::expand_symtabs always passes a
file matcher to expand_symtabs_matching.  However, if 'filenames' is
empty, then this always returns true.  It's slightly more efficient to
pass a null file matcher in this case, because that lets the "quick"
symbol implementations skip any filename checks.

Regression tested on x86-64 Fedora 34.