Modity the :json_data_empty attribute from `:null => false` to `:null => true`

to address ORA-01400 errors with Oracle enhanced adapter.

The original commit 3c0bf043 requires :json_data_empty attribute
has empty string OR null, then setting `:default => ""` is enough.

activerecord/test/schema/schema.rb

@@ -42,7 +42,7 @@ def create_table(*args, &block)
     # big varchar below.
     t.string :preferences, :null => true, :default => '', :limit => 1024
     t.string :json_data, :null => true, :limit => 1024
-    t.string :json_data_empty, :null => false, :default => "", :limit => 1024
+    t.string :json_data_empty, :null => true, :default => "", :limit => 1024
     t.references :account
   end