Auto composition length adjustment in Media Encoder

Hello everyone,

Recently I need to upload many .pdfs in a video form. I prepared my workflow in After Effects which has 64 jpeg files (pdf to jpeg) - the composition 10 mins... My problem is that so often are fewer jpeg files after all. So the question is it possible to establish auto (time?) adjustment in Media Encoder... That the software could after detecting for example missing files just adjust the video length? Or something similar? I set some Watch Folders which work perfectly fine, but adjusting the length in Media encoder it's time-consuming... Daily I'm working on +- pdfs... Is there any chance how to solve it? 

That's set by me:

The missing file starts here: