Gracefully deal with empty ossync files in UrlSynchronization

modules/sync/syncs/urlsynchronization.cpp

@@ -210,7 +210,7 @@ bool UrlSynchronization::isEachFileValid() {
         file.close();
         createSyncFile();
         // File is valid until some date
-        return true; 
+        return true;
     }
     // Otherwise first line is the version number.
     std::string ossyncVersion = line;
@@ -244,6 +244,10 @@ bool UrlSynchronization::isEachFileValid() {
         // i.e. no sync needed.
         return fileValidAsJ2000 > todaysDateAsJ2000;
     }
+    else if (ossyncVersion.empty()) {
+        // For some reason we ended up with an empty synchronization file
+        return false;
+    }
     else {
         LERROR(fmt::format(
             "{}: Unknown ossync version number read. Got {} while {} and below are valid",