Identify YamlFile by path, not name

There is no point in storing a 'name' for a YamlFile, let's use the
path, which is unique.

Also, adds a test for the property.

Signed-off-by: martin f. krafft <madduck@madduck.net>
Handle reading of empty YamlFiles

python-yaml's safe_load returns 'None' when it parses an empty YamlFile,
but since empty YamlFiles correspond to empty dictionaries, really, we
handle things accordingly.

Signed-off-by: martin f. krafft <madduck@madduck.net>
Test applications parsing in YamlFile

Signed-off-by: martin f. krafft <madduck@madduck.net>