Fix omit empty for amino

[Zetazzz]

Also need to consider how to generate default amino string field:

  string memo = 8;

should be generated to:

memo = memo;

or

memo = memo === "" ? undefined : memo

[rasoulMrz]

Also need to consider how to generate default amino string field:

  string memo = 8;

should be generated to:

memo = memo;

or

memo = memo === "" ? undefined : memo

Always need to omit empty fields, unless we have the annotation stating otherwise. So the second one is the right one for toAmino.