Throw exception on empty filepath in create_folder
Throw exception on empty filepath in create_folder

Now, when securesystemslib.storage.FilesystemBackend.create_folder()
is called with an empty string it returns immediately,
without exception, but if we call os.mkdir() with an empty string
we receive this exception:
"[Errno 2] No such file or directory: ''

If we want to mimic the os.mkdir() function when using the
securesystemslib.storage.FilesystemBackend.create_folder()
the function we need to throw our respective exception
securesystemslib.exceptions.StorageError with an appropriate message
when an empty path is given.

Signed-off-by: Martin Vrachev <9a435b9cd6675a0f8cece6f0d3314da6092e8578@vmware.com>

secure-systems-lab/securesystemslib,secure-systems-lab/securesystemslib