explicitly add `DEFAULT NULL` if is_nullable and defalt_value not specified.
trivial change
Checking in changes prior to tagging of version 0.10.

Changelog diff is:

diff --git a/Changes b/Changes
index 21df93a..f888226 100644
--- a/Changes
+++ b/Changes
@@ -2,6 +2,9 @@ Revision history for Perl extension DBIx::Schema::DSL

 {{$NEXT}}

+0.10 2013-12-19T07:24:59Z
+    - explicitly add `DEFAULT NULL` if `is_nullable` and not specified `default_value`
+
 0.09 2013-11-06T06:14:20Z
     - suppress warn message of tinyint