Fix empty checks

[tcrouch]

Consistently order calls to nil? & empty?, as empty? is undefined for NilClass.

[timdiggins]

This looks good -- or is not shortcircuiting because lower precedence -- it's not a boolean operator really (http://www.virtuouscode.com/2010/08/02/using-and-and-or-in-ruby/)

In the test suite this changes a "no implicit conversion of nil into String" error in the codebase into a RestClient::Unauthorized: 401 Unauthorized (which is progress).

Can it be merged?

Alternatively could use &.empty? if you don't mind excluding < ruby 2.3 (which seems like a good shout as ruby 2.2 is EOL https://www.ruby-lang.org/en/downloads/branches/)

Would need to document that somewhere (also a good shout).