When calculating per-DC tablets the formula is shards_in_dc / rf_in_dc,
but the denominator in it can be configured to be literally zero and the
division doesn't work.
Fix by assuming zero tablets for dcs with zero rf
fixes: #16844
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
Closesscylladb/scylladb#16861