Using Optional Fields in Claygent Prompts for Search Queries

Within a Claygent prompt, how can one prompt the engine to use a field in its search, but only if that field is filled in? It is just another point of information that may be referenced to help validate, but it is not always filled. When I try to get the prompt to run, most of the time it returns "some inputs missing" where the field i've said is optional is blank.