journal: return better error for empty files

When reading stuff, we should only return EIO when an actual read error
occured, not when we don't like the data for whatever reason.

We already return ENODATA for all other kinds of file truncation, hence
do the same for the most obvious kind, so that callers know what ENODATA
means.

src/journal/journal-file.c

@@ -2705,7 +2705,7 @@ int journal_file_open(
         }
 
         if (f->last_stat.st_size < (off_t) HEADER_SIZE_MIN) {
-                r = -EIO;
+                r = -ENODATA;
                 goto fail;
         }