Mysql validator crashes with None db_type return. - Backport of master fix to 1.7

[Adam DePue]

When implementing a custom field type with a db_type() method that returns None, on Django 1.6 and below we would signify not to create a column at all.
With Django 1.7, this causes a crash with the mysql backend.
The issue is already fixed in master and this simply backports the safety check.

A fix was made in August in master:
​https://github.com/django/django/commit/e9103402c0fa873aea58a6a11dba510cd308cb84#diff-14

The fix contains a lot more than is needed for this specific issue, however the specific changes to django/db/backends/mysql/validation.py would be great to have in Django 1.7.

This worked fine in < Django 1.7 and works fine in upstream.

[Adam DePue]

Backport fix PR'd here:
​https://github.com/django/django/pull/3471

[Tim Graham]

A regression test and a mention in the 1.7.2 release notes is also required.

[Adam DePue]

PR is updated to include mention in release notes as well as a regression test.
Updated PR here:
​https://github.com/django/django/pull/3472

[Tim Graham <timograham@…>]

In 2d12a599383efff4ac1914e4cc5cf3b08cb2ff53:

[1.7.x] Fixed #23761 -- Fixed crash with MySQL validator and db_type is None.

The issue was fixed on master in e9103402c0fa873aea58a6a11dba510cd308cb84.

[Tim Graham <timograham@…>]

In e118e3af371ae58cee7420d2ea2020c8bdeb461e:

Fixed #23761 -- Added test for MySQL validator and db_type is None.

The issue was fixed on master in e9103402c0fa873aea58a6a11dba510cd308cb84
so this just forwardports the test and release notes from stable/1.7.x.

Forwardport of 2d12a59938 from stable/1.7.x