Memo fields have two ways to encode an empty memo:
- 0xF6 followed by all-zeroes, encoding "there is no memo".
- All-zeroes, encoding the empty UTF-8 string.
In almost all cases you want the former, but users thinking about byte
slices may expect MemoBytes::default() to result in the latter. To
ensure clarity, we now require calling either MemoBytes::default() or
MemoBytes::from_bytes(&[]) to be explicit.
No such confusion exists for the Memo enum, because the two types are
visibly separated as different enum cases, and Memo::Empty makes sense
as the default.
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