Fix -Wnonnull-compare warning
[1/4] Compiling C object tls/tests/connection-gnutls.p/connection.c.o
../../../../Projects/glib-networking/tls/tests/connection.c: In function ‘clock_gettime’:
../../../../Projects/glib-networking/tls/tests/connection.c:600:16: warning: ‘nonnull’ argument ‘tp’ compared to NULL [-Wnonnull-compare]
600 | if (ret == 0 && tp)
| ~~~~~~~~~^~~~~