If incoming sysex is just "F0 F7" with no data, file is created, but shows error message and isn't added to library

[krevis]

Have a user with a no-name USB-MIDI cable which appears to be dropping sysex data. We're getting only F0 F7 but with no data inside.

When this happens, we save the .syx file (containing one or more iterations of F0 F7). However when we go to read in the contents of that file, we are hitting an error path, and show an alert that "The file could not be created".

Either we should filter out sysex messages with no contents earlier in the recording path, or we should treat these zero-contents messages as being allowable. Probably the former. Perhaps tell the user when we get them, if this is a common enough thing.