Can TrieDateField fields be null?

[Henrique O. Santos]

Hello,

Just a simple question. Can TrieDateField fields be null? I have a 
schema with the following field and type:
    <field name="started_at" type="date" indexed="true" stored="true" 
docValues="true" />
    <fieldType name="date" class="solr.TrieDateField" precisionStep="0" 
positionIncrementGap="0"/>

Every time I index a document with no value for this field, the current 
time gets indexed and stored. Is there anyway to make this field null?

My use case for this collection requires that I check if that date field 
is already filled or not.

Thank you,
Henrique.