Fix Validation failed error using sshkey --upload

when using `travis sshkey --upload a_file` without a passpharse error ::
  
  Validation failed (value: missing field, value: not a private key)

is raised as rsa_key is empty due to wrong empty return value
Merge pull request #222 from nbessi/fix_update_wihout_passphrase

Fix Validation failed error using sshkey --upload