The DXF extension creates an invalid DXF if the input feature collection is empty.
The might be due to a bbox comprising no features.
An geometric envelop is calculated, but the envelop is null internally. The calculation of the aspect ration fails (division by zero) and a none valid aspect ratio is written to the DXF.
The DXF is broken then.
BTW: There is no component "DXF" to choose, so I selected "Main". Maybe anyone can correct this.