hash_incrementals script hangs on sha256sum when no .incremental.mar files are present

Ran into this today when attempting to build tbb-11.5a4-build1

I had incorrectly set torbrowser_incremental_from to 11.5a3 which was an Android-only release. As 11.5a4 is Desktop-only, there were no .incremental-mar files to be used.

We should verify that the set of filenames output by ls -1 | grep '\.incremental\.mar$' | sort is not empty, otherwise sha256sum will hang indefinitely waiting for data to be read in from stdin.