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Add sortMissingLast="true"
to the field that shall have the behaviour. After that ordering ascending or descending by that field will put the null values last. As far as I know this change will require you to re-index, before it takes effect.
<field name="sort_date" type="date" indexed="true" stored="true"
multiValued="false" sortMissingLast="true" />
<field name="sort_date_exact" type="date" indexed="true" stored="true"
multiValued="false" sortMissingLast="true" />
There have been some issues prior to Solr 3.5, which you can read up in the related question Sorting dates using Solr is messing up some dates. As I do not know which version you use, this could be of interest.
Source:stackexchange.com