Support empty <format specific parameters> in a=fmtp: line

[garethsb]

Proposed fix for #14.

Note that there's still more to do to bring ST 2110-40 testing up to same level as ST 2110-20, see #4.
And #11 is required ASAP as well, now the ST 2110-xx:2022 revisions are imminent.

[garethsb]

Note that there's still the existing bug that if the a=fmtp: line appears before the a=rtpmap: line, e.g. test_20_72_1 fails because extractMTParams doesn't work out it needs to skip the format specific parameter checks in time. Luckily, all files I've seen in the wild order rtpmap before fmtp.

[pedro-alves-ferreira]

LGTM