log: set empty string to not set log_fields

not all functions in glib returns a value for the used log fields in this case a 'g_return_val_if_fail(..) will only set 4 fields and set the other field to NULL which leads to a segfault

Jul 22 13:41:37 Cynap-Pro-01121508 systemd-coredump[7247]: Process 496 (wireplumber) of user 4242 dumped core.

                                                       Stack trace of thread 496:
                                                       #0  0x00007f6b2ce58e99 __strlen_avx2 (libc.so.6 + 0x150e99)
                                                       #1  0x00007f6b2cf3ebe5 g_log_writer_journald (libglib-2.0.so.0 + 0x5fbe5)
                                                       #2  0x00007f6b2d0a9316 wp_log_fields_write_to_journal (libwireplumber-0.5.so.0 + 0x2b316)
                                                       #3  0x00007f6b2d0a96f5 wp_log_writer_default (libwireplumber-0.5.so.0 + 0x2b6f5)
                                                       #4  0x00007f6b2cf3cf3e g_log_structured_array (libglib-2.0.so.0 + 0x5df3e)
                                                       #5  0x00007f6b2cf3d0ae g_log_structured_array (libglib-2.0.so.0 + 0x5e0ae)
                                                       #6  0x00007f6b2cf3d4b7 g_logv (libglib-2.0.so.0 + 0x5e4b7)
                                                       #7  0x00007f6b2cf3d7ef g_log (libglib-2.0.so.0 + 0x5e7ef)
                                                       #8  0x00007f6b29efafef get_node_by_id (libwireplumber-module-cb-ipc.so + 0x24fef)
                                                       #9  0x00007f6b29efb032 wp_cb_ipc_get_app_properties (libwireplumber-module-cb-ipc.so + 0x25032)