ccconverter: fix missing output framerate on the caps

ccconverter: fix missing output framerate on the caps

A pipeline like this:

closedcaption/x-cea-708,format=cdp,framerate=30000/1001 ! ccconverter ! closedcaption/x-cea-708,format=cc_data

would produce a critical/assert:

GStreamer-CRITICAL **: 14:21:11.509: gst_util_fraction_multiply: assertion 'a_d != 0' failed

because there would be no framerate field on ccconverter's output.

Fixed by always fixating a framerate if the input has a framerate.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1393>