gdbarch-components.py: change empty "params" tuples to empty lists

During review, it was suggested to change the "params" parameter from a
tuple to a list, for esthetic reasons.  The empty ones are still tuples
though, they should probably be changed to be empty lists, for
consistency.  It does not change anything in the script result.

Change-Id: If13c6c527aa167a5ee5b45740e5f1bda1e9517e4