rtphdrext: make input_meta nullable

Most extensions do not do anything with the input_meta buffer in their get_max_size / write implementations, and some payloaders will push out packets before they've processed anything.

An extension like twcc should ideally extend every packet that comes out for accuracy purposes, and this behavior was also breaking support for protecting extension data in the rtpst2022-1 FEC encoder (!2224):

The specification requires that if input packets contain extension data, they should all have the extension bit set, and the extension data should always have the same size. Not adding the extension data for the first few packets with rtph265pay was breaking this requirement for no good reason.