[#459] Omit empty lists in the json representation of `Expression`

Description

Problem: When serializing large Micheline Expressions, we can get really bloated JSONs, with lots of annots and args fields with empty lists.

Due to the introduction of TZIP-16, which says views should be stored as micheline expressions (possibly inside the contract's storage), we now have to be very careful to produce compact JSON values.

Solution: Change the ToJSON instance of MichelinePrimAp to omit annots and args when these lists are empty.

Related issue(s)

Resolves #459 (closed)

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