[1.6.x] Fixed #22145 -- Clarified docs on Field.blank and Field.null options.

If ``True``, Django will store empty values as ``NULL`` in the database. Default

is ``False``.

Note that empty string values will always get stored as empty strings, not as

``NULL``. Only use ``null=True`` for non-string fields such as integers,

booleans and dates. For both types of fields, you will also need to set

``blank=True`` if you wish to permit empty values in forms, as the

:attr:`~Field.null` parameter only affects database storage (see

:attr:`~Field.blank`).

Avoid using :attr:`~Field.null` on string-based fields such as

:class:`CharField` and :class:`TextField` unless you have an excellent reason.

If a string-based field has ``null=True``, that means it has two possible values

for "no data": ``NULL``, and the empty string. In most cases, it's redundant to

have two possible values for "no data;" Django convention is to use the empty

string, not ``NULL``.

:class:`CharField` and :class:`TextField` because empty string values will

always be stored as empty strings, not as ``NULL``. If a string-based field has

``null=True``, that means it has two possible values for "no data": ``NULL``,

and the empty string. In most cases, it's redundant to have two possible values

for "no data;" the Django convention is to use the empty string, not ``NULL``.

For both string-based and non-string-based fields, you will also need to

set ``blank=True`` if you wish to permit empty values in forms, as the

:attr:`~Field.null` parameter only affects database storage

(see :attr:`~Field.blank`).

.. note::