Fix count_where() raising OperationalError on nonexistent tables

rows_where() and delete_where() already return gracefully when called
on a table that does not exist yet; count_where() now does the same by
returning 0 instead of propagating OperationalError from SQLite.

sqlite_utils/db.py

@@ -2023,6 +2023,8 @@ class Queryable:
         :param where_args: Parameters to use with that fragment - an iterable for ``id > ?``
           parameters, or a dictionary for ``id > :id``
         """
+        if not self.exists():
+            return 0
         sql = f"select count(*) from {quote_identifier(self.name)}"
         if where is not None:
             sql += " where " + where