Refs #33288 -- Made SQLite introspection raise DatabaseError on nonexistent tables.

All the other backends behave this way and we had to make adjustments
to our test suite to account for this discrepancy.

This also allows SQLite's get_relations() not to raise on a nonexistent
table while making sure the InspectDBTestCase.test_introspection_errors
test which ensures an error during introspection is surfaced in
generated models files still passes.

tests/schema/tests.py

@@ -119,9 +119,6 @@ class SchemaTests(TransactionTestCase):
         for name, (type, desc) in columns.items():
             if isinstance(type, tuple):
                 columns[name] = (type[0], desc)
-        # SQLite also doesn't error properly
-        if not columns:
-            raise DatabaseError("Table does not exist (empty pragma)")
         return columns
 
     def get_primary_key(self, table):