Fix mockPrompt's handling of falsy answers

Fix `mockPrompt` and `DummyPrompt` to copy Inquirer's behaviour when
dealing with falsy answers.

 - The `list` prompt treats all values including `null` and `false`
   as a valid response.

 - All other prompts are kept unchanged, they replace every falsy
   answer with the default value.
Merge pull request #711 from strongloop-forks/fix/dummy-prompt

Fix mockPrompt's handling of falsy answers