gstv4l2tuner: return NULL if no norm set

If the video4linux device supports norms but has no norm set, norm is
returned as an uninitialized variable after the ioctl call, leading to
gst_v4l2_tuner_get_norm_by_std_id() returning a random norm from the
supported norms. Catch this case and instead return NULL to indicate
that no norm is setup.

Part-of: <gstreamer/gstreamer!1625>
24 jobs for !1625 with fix/random_norm_if_no_norm in 52 minutes and 56 seconds (queued for 1 second)
merge request