Add support for selecting empty string as null

[SashaKalko]

When we have an empty string in csv, and this field is date type in elastic search 1.7 we get an exception from ES :
([{u'create': {u'status': 400, u'_type': u'csv_type', u'_id': u'AVgBSdxaLywLgoALVw4Q', u'error': u'MapperParsingException[failed to parse [ticket_metrics_created_at_hour]]; nested: MapperParsingException[failed to parse date field [], tried both date format [YYYY-MM-DD HH:mm:ss], and timestamp number with locale []]; nested: IllegalArgumentException[Invalid format: ""]; ', u'_index': u'test_0_delete'}}

[jaune-rouge]

elasticsearch 5.4 still an issue
I set ignore_malformed to true