Always return dict for custom metadata

[dennisvang]

Always return a dict from TargetMeta.custom and custom_internal (instead optional dict).

This breaks any code that explicitly relies on the possibility that custom metadata is None, for example:

if my_meta.custom is None:
    # this will never happen...
    ...

Code that simply checks for custom metadata, as in if my_meta.custom: ..., or if my_meta.custom is not None: ..., is not affected.

fixes #131