Fix failing unit tests

Corresponding to the securesyslib changes made in this pr:
secure-systems-lab/securesystemslib#244
which changed the securesyslib.formats.PATH schema to be of type
AnyNonemptystring.
This made the tuf unit tests to fail because there are to places
where functional arguments should comply with the
securesyslib.formats.PATH schema, but have a default value of
an empty string.

Signed-off-by: Martin Vrachev <[email protected]>

tuf/developer_tool.py

@@ -584,7 +584,7 @@ def create_new_project(project_name, metadata_directory,
   # Do the same for the location in the repo and the project name, we must
   # ensure they are valid pathnames.
   securesystemslib.formats.NAME_SCHEMA.check_match(project_name)
-  securesystemslib.formats.PATH_SCHEMA.check_match(location_in_repository)
+  securesystemslib.formats.ANY_STRING_SCHEMA.check_match(location_in_repository)
   securesystemslib.formats.NAME_SCHEMA.check_match(repository_name)
 
   # for the targets directory we do the same, but first, let's find out what
@@ -797,7 +797,7 @@ def load_project(project_directory, prefix='', new_targets_location=None,
   securesystemslib.formats.NAME_SCHEMA.check_match(repository_name)
 
   # Do the same for the prefix
-  securesystemslib.formats.PATH_SCHEMA.check_match(prefix)
+  securesystemslib.formats.ANY_STRING_SCHEMA.check_match(prefix)
 
   # Clear the role and key databases since we are loading in a new project.
   tuf.roledb.clear_roledb(clear_all=True)