Do not hint --skip-unavailable in case only one pkg_spec was used

[m-blaha]

If only one pkg_spec was used with the transaction command and this spec was not found, do not suggest using --skip-unavailable, because if dnf5 skips this single specification, there would be nothing else left, resulting in an empty transaction.

Current output:

❯ dnf5 install ThisDoesNotExist
Updating and loading repositories:
Repositories loaded.
Failed to resolve the transaction:
No match for argument: ThisDoesNotExist
You can try to add to command line:
  --skip-unavailable to skip unavailable packages

Expected output:

❯ dnf5 install ThisDoesNotExist
Updating and loading repositories:
Repositories loaded.
Failed to resolve the transaction:
No match for argument: ThisDoesNotExist