Enforce serialize_empty for repeated fields

[sterliakov]

When serializing a sequence of elements, serialize_empty must be True to prevent data loss and messages incompatible with original protobuf implementation.

Closes #414
Closes #387

[sterliakov]

Sorry, I haven't noticed .pre-commit-config, only black (and poe format) was mentioned in README. (Should I open a separate PR to document this and add setup.py with 2 lines to support editable pip-installs?)

[Gobot1234]

No, don't worry about it, I've fixed poe format in #415 and since this package uses poetry for development we don't need a setup.py to have an editable install

[sterliakov]

Fixed.

[Gobot1234]

Thank you!