sd-journal: make sure it's safe to call sd_journal_process() before the first sd_journal_wait()

In that case we have no inotify fd yet, and there's nothing to process
hence. Let's make the call a NOP.

(Previously, without this change we'd end up trying to read off inotify
fd -1, which is quite a problem... 😢)

(cherry picked from commit 10c4d640)