closedcaption: write 0x00 padding instead of 0x80 in cc_data/cdp
Depending on the exact output format, 0x00 may be a better default for padding than 0x80. 0x00 is the recommended padding value when used in CDP (and cc_data) but is not when used in s334-1a. See CTA-708-E 4.3.5 amd SMPTE 334-1-2007 5.3.2.