Scaffolder: Prevent having pointer field for a type that has a custom type fields

When scaffolding a type with a custom type field:
```
starport s type foo
starport s map bar foo:Foo 
```

The `foo` field will be a pointer.
We should use the `[(gogoproto.nullable) = false]` option in proto field to avoid having a pointer

Generating
```
message Bar {
  string index = 1; 
  Foo foo = 2 [(gogoproto.nullable) = false];
}
```