Change the zipfile virtual table so that the xBestIndex method gives a

Change the zipfile virtual table so that the xBestIndex method gives a
reasonable cost estimate even if no filename is specified.  The missing
filename error continues to be raised in the xFilter method.  Meanwhile,
the more reasonable cost estimate avoids unnecessary wierdness in the query
planner.

FossilOrigin-Name: 0e468aa6decc73be307596782a778a739ff1b5dc0eb10fc4bd6776e460f99f8b