Fixed #4136 -- Made ModelForm save empty values for nullable CharFields as NULL.

Previously, empty values were saved as strings.

docs/ref/forms/fields.txt

@@ -361,7 +361,7 @@ For each field, we describe the default widget used if you don't specify
 .. class:: CharField(**kwargs)
 
     * Default widget: :class:`TextInput`
-    * Empty value: ``''`` (an empty string)
+    * Empty value: Whatever you've given as :attr:`empty_value`.
     * Normalizes to: A Unicode object.
     * Validates ``max_length`` or ``min_length``, if they are provided.
       Otherwise, all inputs are valid.
@@ -380,6 +380,12 @@ For each field, we describe the default widget used if you don't specify
        If ``True`` (default), the value will be stripped of leading and
        trailing whitespace.
 
+    .. attribute:: empty_value
+
+       .. versionadded:: 1.11
+
+       The value to use to represent "empty". Defaults to an empty string.
+
 ``ChoiceField``
 ---------------
 

docs/ref/models/fields.txt

@@ -43,11 +43,13 @@ If ``True``, Django will store empty values as ``NULL`` in the database. Default
 is ``False``.
 
 Avoid using :attr:`~Field.null` on string-based fields such as
-: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
+:class:`CharField` and :class:`TextField`. 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 "no data;" the Django convention is to use the empty string, not
+``NULL``. One exception is when a :class:`CharField` has both ``unique=True``
+and ``blank=True`` set. In this situation, ``null=True`` is required to avoid
+unique constraint violations when saving multiple objects with blank values.
 
 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