vzdump: included_guest: return empty hash if no guests selected

vzdump: included_guest: return empty hash if no guests selected

This will fix the behaviour when calling `vzdump --stop` to cause all
local guests to be backed up.

When refactoring this logic in commit be308647, the assumption was that
every call will have one of the following parameters set: pool, list of
VMIDs or all (intentional or when exclude is used).

There is an addtional possibility, that vzdump is called with only
--stop. Thus there are no other parameters that would indicate which
VMIDs to include.

In this case we want to return the empty hash.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>