individual checks before calling `inject*` methods

[pine3ree]

This could be considered just as a tiny refactoring / code-style change: no bugs, no new features

1. don't call an inject* method with missing/empty related config values
2. check for empty routes in injectRoutesFromConfig before checking is_array and running the loop on an empty array (this make more sense if the method is called outside this delegator, which, now, already checks for emptiness before calling it)

[weierophinney]

Thanks, @pine3ree.