rustdoc_json: represent generic args consistently.

They show up in three places: once as `Option<Box<GenericArgs>>`, once
as `Box<GenericArgs>`, and once as `GenericArgs`. The first option is
best. It is more compact because generic args are often missing. This
commit changes the latter two to the former.

Example output, before and after, for the `AssocItemConstraint` change:
```
{"name":"Offset","args":{"angle_bracketed":{"args":[],"constraints":[]}},"binding":{...}}
{"name":"Offset","args":null,"binding":{...}}
```
Example output, before and after, for the `Type::QualifiedPath` change:
```
{"qualified_path":{"name":"Offset","args":{"angle_bracketed":{"args":[],"constraints":[]}}, ...}}
{"qualified_path":{"name":"Offset","args":null, ...}}
```
This reduces JSON output size, but not by much (e.g. 0.5%), because
`AssocItemConstraint` and `Type::QualifiedPath` are uncommon.