crypto: Avoid pretending to verify signatures on unsigned encrypted mail

crypto: Avoid pretending to verify signatures on unsigned encrypted mail

Unsigned encrypted mail shows up with a weird empty signature list.
If we successfully decrypted and there was no signature in it, we
should just not show a sigstatus at all.

The documentation for g_mime_decrypt_result_get_signatures says:

    a GMimeSignatureList or NULL if the stream was not signed.