[llvm-readelf] - Improve dumping of objects without a section header string table.

We have a test/Object/no-section-header-string-table.test which checks
what happens when an object does not have a section header string table.
It does not check the full output though.
Currently our output is different from GNU readelf, because the latter prints
"<no-strings>" instead of a section name, while we print nothing.

This patch fixes this, adds a proper test case and removes the one from test/Object,
as it is not a right folder for llvm-readelf tests.

Differential revision: https://reviews.llvm.org/D73193