Fix Validation failed error using sshkey --upload
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
deivid-rodriguez/travis.rb,vinaykaradia/rubyBuildClone,prabagithub/travis.rb,travis-ci/travis.rb,danishkhan/travis.rb,Zearin/travis.rb,deivid-rodriguez/travis.rb,prabagithub/travis.rb,snordhausen/travis.rb,danishkhan/travis.rb,deivid-rodriguez/travis.rb,vinaykaradia/rubyBuildClone,doct15/travis.rb,whip112/travis.rb,Tiger66639/travis.rb,Zearin/travis.rb,snordhausen/travis.rb,travis-ci/travis.rb,whip112/travis.rb,snordhausen/travis.rb,chriskuehl/travis.rb,chriskuehl/travis.rb,doct15/travis.rb,doct15/travis.rb,Tiger66639/travis.rb,Zearin/travis.rb,chriskuehl/travis.rb,danishkhan/travis.rb,prabagithub/travis.rb,whip112/travis.rb,travis-ci/travis.rb,Tiger66639/travis.rb,vinaykaradia/rubyBuildClone