Before this commit, both writing and reading an encoded empty set of
short channel IDs from the wire would fail. Prior to this commit, we
treated decoding an empty set as a caller error, and failed to write out
the zlib encoding of an empty set in a way that us and the other
implementations were able to read.
To fix this, rather than giving zlib an empty buffer to write out (which
results in an encoding with the zlib header data and the rest), we just
write a blank slice. When decoding, if we have an empty query body, then
we'll return a `nil` slice.
With the above changes, we'll now always write out an empty short
channel ID set as:
```
0001 (1 byte follows) || <encoding_type>
```
A new test has also been added to exercise this case for both known
encoding types.
