xdg-autostart: ignore all empty entries in multi-string entries

xdg-autostart: ignore all empty entries in multi-string entries

The desktop file specification allows entries like ";;;;;;", full of empty strings.
But looking at the actual list of supported keys [1], empty entries are meaningless
(unless we would allow e.g. the desktop name to be the empty string. But that doesn't
seem very useful either). So let's just simplify our life and skip any empty substrings
entirely.

This would also resolve the fuzzer case:
$ valgrind build/fuzz-xdg-desktop test/fuzz/fuzz-xdg-desktop/oss-fuzz-22812
test/fuzz/fuzz-xdg-desktop/oss-fuzz-22812... ok
==2899241== HEAP SUMMARY:
==2899241==     in use at exit: 0 bytes in 0 blocks
==2899241==   total heap usage: 484,385 allocs, 484,385 frees, 12,411,330 bytes allocated
↓
==2899650== HEAP SUMMARY:
==2899650==     in use at exit: 0 bytes in 0 blocks
==2899650==   total heap usage: 1,325 allocs, 1,325 frees, 1,463,602 bytes allocated