[patch]: Unique_for_date fails when the required DateTimeField is left empty in the admin

[remco@…]

When the publicationDate field in the following model is left empty then the unique_for_date validator function receives an empty date string, in this case the unique_for_date should leave validation up to the publicationDate field (see patch)

class Article(Model):

publicationDate = DateTimeField("Publicatie datum")
linkTitle = SlugField("Link titel", prepopulate_from=("title",), unique_for_month='publicationDate')

[Malcolm Tredinnick]

This was fixed in [3277], but somehow I messed up the "auto-close" portion of the commit message. For the record, the commit message was:

Fixed #1980, #2155 -- made date, time and datetime html2python methods a bit more consistent in their return values. All three now return None when the passed in string cannot be converted to the required object (this is assumed elsewhere).