Error if the user has not entered a Project Root PHP Namespace in the set-up prompt

[mimikim]

On the master branch, if the user runs yo wp-make:theme and breezes through the set-up prompt, everything else is set to a default value except for the root namespace.

We should either enable a default value in the setup conditional statement, or run a check to make sure opts.root_namespace is not empty before generating the php, otherwise we have a random errant backslash causing errors