Separate Task to check for empty (0-byte) files

[pjotrek-b]

There's already a check for 0-byte files in Task "FilesValid" (see #52).

However, in cases where files are created from Tasks, it sometimes happened that 0-Byte files were generated (by custom Pre/Postproc scripts) that were in fact empty.

This usually ways an error of the called program, but wasn't always caught / catchable.

So: Having the option to error on invalid files more modular in the Tasklist is preferred.

[pjotrek-b]

Idea: Similar to preproc/postproc, create a new thin instance "FilesValid[234]" of "FilesValid", to allow the same behavior (and code) - but multiple times.

Would also lead to FILES_VALID2[], etc options for granular config.