Resolve a zero-length BlockAddID to its default of 1.

BlockAdditions are parsed outside the element descriptors, and that manual parse overwrote the preset default with the zero ne_read_uint returns for a zero-length element, which the following add_id == 0 check then rejected: a zero-length BlockAddID failed the whole packet read rather than resolving to 1, the default RFC 9559 Section 5.1.3.5.2.3 declares for it. Read the value only when the element is not empty. libavformat’s matroskadec defaults the same element to 1. Add zerolen_blockaddid.webm.