MySQL driver has incorrect primary key default, causing failure to insert auto increment rows with no additional values.

[Peter Venable]

The problem:

Given a MySQL table with only one column, an auto-increment primary key integer, inserting a row should be as simple as calling MyTable.objects.create()
However, the generated SQL is:

INSERT INTO `myapp_mytable` (`id`) VALUES (DEFAULT);
-- Error Code: 1062. Duplicate entry '0' for key 'PRIMARY'

Instead it should generate:

INSERT INTO `myapp_mytable` (`id`) VALUES (NULL);

The fix could be a one-line edit to ​https://github.com/django/django/blob/master/django/db/backends/base/operations.py#L294 changing 'DEFAULT' to 'NULL'.