How can I map a null_value for a date field?
Pretty sure you cannot do that - https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-date-format.html
I'm not sure if I completely understand what you are after, but if you want to insert a particular value when a document has a value of null for a date field have a look at the null_value option here: https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-core-types.html#date
If that's not what you are after please could you elaborate on what you are wanting to do?