CLI: Do not display message about the empty keyrings if generate command is given

[ni4]

Description

It's mostly cosmetical issue, but still worth fixing:

$ rnpkeys --homedir .rnp --generate
Keyring directory '.rnp' is empty.
Use "rnpkeys" command to generate a new key or import existing keys from the file or GnuPG keyrings.
Generating a new key...