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

tests/test_introspect.py

@@ -96,6 +96,12 @@ def test_count_where(existing_db):
     assert existing_db.table("foo").count_where("text != :t", {"t": "two"}) == 2
 
 
+def test_count_where_nonexistent_table(fresh_db):
+    assert fresh_db.table("does_not_exist").count_where() == 0
+    assert fresh_db.table("does_not_exist").count_where("id > ?", [5]) == 0
+    assert fresh_db.table("does_not_exist").count == 0
+
+
 def test_columns(existing_db):
     table = existing_db.table("foo")
     assert [{"name": "text", "type": "TEXT"}] == [