Fix unexpected behavior of --lock when keepassxc is not running (#8889)

currently, when keepassxc is not running, the command `keepassxc --lock` opens a new keepass window and blocks until the window is closed.

Especially in locking scripts this is rather unexpected and  Ican't think of a case where someone explicitly starts keepass with --lock and wants this behaviour.

Rather --lock should always ensure, that there are no unlocked instances and exiting afterwards

src/main.cpp

@@ -156,6 +156,14 @@ int main(int argc, char** argv)
         return EXIT_SUCCESS;
     }
 
+    if (parser.isSet(lockOption)) {
+        qWarning() << QObject::tr("KeePassXC is not running. No open database to lock").toUtf8().constData();
+
+        // still return with EXIT_SUCCESS because when used within a script for ensuring that there is no unlocked
+        // keepass database (e.g. screen locking) we can consider it as successful
+        return EXIT_SUCCESS;
+    }
+
     if (!Crypto::init()) {
         QString error = QObject::tr("Fatal error while testing the cryptographic functions.");
         error.append("\n");