Generate SVM manifest file even for cases where there were no viable images

[mdlpstsci]

According to Rob..

Our workflow creates a trigger file for each flc or flt file. The poller picks up those triggers. When it has received all triggers for the visit, it will trigger the HAP single visit mosaic workflow. Do grism data end up with flc/t files? If not, then they’ll never make it to the manifest file.

It is possible that a visit has no viable images (e.g., all grism images) for the generation of a mosaic, but the Grism data do have FLT/FLC images. As such we should ensure the trailer filenames for these images are entered into the manifest file.