[BUG] Check for 0-length module files

[EricClaeys]

If a file in ~/allsky/config/modules is 0-length, the following errors appear in the log file ever image:

ERROR: Error parsing /home/rpi/allsky/config/modules/postprocessing_night.json Expecting value: line 1 column 1 (char 0)
ERROR: Failed to open /home/rpi/allsky/config/modules/postprocessing_night.json

I suggest having the code explicitly check for non-existent and 0-length files and issue a single, meaningful message.

Further, the Module Manager should also perform sanity checks like this and warn the user, since most users don't look in the log file.