Type of ImageFileCollection summary property

[MSeifert04]

Currently it can be a Table or None (no files) or an empty list (no keywords). Internally it's also temporarily (shouldn't be visible to users) set to an empty dictionary. However the docs state that it's a Table.

Personally I would prefer if it would always be a Table, however that would be backwards-incompatible, especially for checks regarding if collection.summary tests (Tables are always True).

Does it make any sense to change the behavior here?

[mwcraig]

How about this:

• No keywords returns a table with one column (files)
• No files returns None (could be talked into a table of length zero instead)

[mwcraig]

Will implement as above