fixed decoder behavior when nbSeqs==0 is encoded using 2 bytes

fixed decoder behavior when nbSeqs==0 is encoded using 2 bytes

The sequence section starts with a number, which tells how sequences are present in the section.
If this number if 0, the section automatically ends.

The number 0 can be represented using the 1 byte or the 2 bytes formats.
That's because the 2-bytes formats fully overlaps the 1 byte format.

However, when 0 is represented using the 2-bytes format,
the decoder was expecting the sequence section to continue,
and was looking for FSE tables, which is incorrect.

Fixed this behavior, in both the reference decoder and the educational behavior.

In practice, this behavior never happens,
because the encoder will always select the 1-byte format to represent 0,
since this is more efficient.

Completed the fix with a new golden sample for tests,
a clarification of the specification,
and a decoder errata paragraph.