I implemented an additional check, like bigints and indexes checks, for Admin -> Miscellaneous -> XMLDB Editor.
It for 1.9.x.
Not for 2.0!
The check fixes fields that are NULL in the actual db and defined as NOT NULL in XMLDB definitions.
Never vice-versa (NOT NULL to NULL).
It also adds missing DEFAULTs to fields.
It does not remove existing DEFAULTs, nor changes DEFAULTs different from defined in XMLDB.
I excluded some cases for robustness.
I used the messages I found in standard lang files for this check.
This check was probably planned but not implemented (I suppose).
They should be slightly changed to reflect the actual behaviour.
The attached zip contains 2 files.
One defines a new check_default XMLDBAction.
The second replaces main_view action, just adding the new check to the XMLDB Editor page.
Files should be unzipped in the main installation directory.
I tested it on MySQL and PostgreSQL (it is disabled for other DBMS).
I suggest not to use it on production sites without some tests. Consider it experimental.
Please let me know if you find any problem.