Snapshot and Timestamp default content

Snapshot and Timestamp constructors try to be clever:

self.meta = meta if meta is not None else {"targets.json": MetaFile(1)}

and

self.snapshot_meta = snapshot_meta or MetaFile(1)

So they set the metafile content without knowing what it really should be

this is annoying as Repository.snapshot() and Repository.timestamp() now think a snapshot and timestamp are not needed -- even though none have been generated yet.

I can work around this but especially for snapshot the default value seems wrong: empty dict would be more correct -- the meta dict should be filled by a conscious decision not by a default value that might be right.

For timestamp there might not be a correct value though. I think MetaFile(0) would be best (although it requires loosening the check for valid MetaFiles): it's at least clear that timestamp doesn't have a snapshot version yet

Start by reading the Snapshot and Timestamp constructors and the Repository.snapshot() and Repository.timestamp() methods to understand how their default metadata affects generation checks. Compare the proposed empty-dict and MetaFile(0) defaults with the existing MetaFile validity rules; done means newly constructed objects do not falsely indicate that snapshot or timestamp metadata has been generated.