SQLExpression: only warn for string from_expression

EntireQuery pass a PlaceHolderTableDefinition for almost all catalog
searches, so "from_expression" itself can not be deprecated at this
point

product/ZSQLCatalog/SQLExpression.py

@@ -133,7 +133,7 @@ class SQLExpression(object):
         raise ValueError('Unrecognized "limit" value: %r' % (limit, ))
     else:
       self.limit = (limit, )
-    if from_expression is not None:
+    if isinstance(from_expression, basestring):
       warnings.warn("Providing a 'from_expression' is deprecated.",
                     DeprecationWarning)
     self.from_expression = from_expression