org.el: Fix setting and deleting properties with a null value

org.el: Fix setting and deleting properties with a null value

* org.el (org-property-re): Also match null properties by
default.
(org-entry-delete): Also delete null properties.
(org-read-property-value): Allow the empty string as a new
value.
(org-delete-property): Throw a message when there is not
property to delete.

Thanks to Andrea Rossetti for reporting this.