gdb/debuginfod: Prevent out_of_range exception

Trailing whitespace in the string of debuginfod URLs causes an
out_of_range exception during the printing of URLs for the first
use notice.

To fix this, stop printing URLs when the substring to be printed
consists only of whitespace.

Also add first use notice testcases.

Co-Authored-By: Pedro Alves <pedro@palves.net>