file_create_path returns inaccurate value if the files directory doesn't exist

If the files directory doesn't exist yet, a call to file_create_path() returns bad data.

- call file_create_path('myfile') when /files doesn't exist: get back 'myfile'
- call file_create_path('myfile') when /files DOES exist: get back 'files/myfile'

The flow is this:
- my module (module_builder) calls file_create_path('hooks')
- file_create_path calls file_check_location('hooks', 'files'), having got the value of 'files' from file_directory_path().
- file_check_location() finds that the location doesn't exist, as it gets back nothing from realpath('hooks').
- file_check_location() rebuilds $source
- $directory = realpath($directory); places an empty string in $directory because 'files' doesn't exist.
- the strpos check isn't carried out
- $source is returned.
- file_create_path ignores this returned value, and returns the unchanged file it was given in the first place.

I'm not sure what you SHOULD get back in if 'files' doesn't exist -- either the same thing as if it did, or NULL? It's possibly up to modules to check this first but it still seems incorrect to return the current value.