Fix crash with list:reference field

When using validate_and_update() or validate_and_insert() on a table with a list:reference field, the request crashes after timeout with:
  File "web2py/gluon/globals.py", line 270, in body
    raise HTTP(400, "Bad Request - HTTP body is incomplete")

The request crashes because there is an hidden exception with the isinstance function:
isinstance() arg 2 must be a class, type, or tuple of classes and types

When no using GAE, the GoogleDatastoreAdapter variable is None, so isinstance crash.
See the last line of pydal/adapters/__init__.py

This is a regression intruduced after the v2.9.12.

gluon/validators.py

@@ -611,7 +611,7 @@ class IS_IN_DB(Validator):
 
                 def count(values, s=self.dbset, f=field):
                     return s(f.belongs(map(int, values))).count()
-                if isinstance(self.dbset.db._adapter, GoogleDatastoreAdapter):
+                if GoogleDatastoreAdapter is not None and isinstance(self.dbset.db._adapter, GoogleDatastoreAdapter):
                     range_ids = range(0, len(values), 30)
                     total = sum(count(values[i:i + 30]) for i in range_ids)
                     if total == len(values):