{{#FieldName}}
    This text is only shown if FieldName has text in it
{{/FieldName}}

{{^FieldName}}
    This text is only shown if FieldName is empty
{{/FieldName}}

This functionality proves particularly handy when generating an “optional card” based on the presence of content in a field. For example, this is how Anki’s “Basic (optional reversed card)” note type work.

References