Add omitempty json tags for nullable fields

[StevenACoffman]

Fix #789: Request for omitempty on json struct tag for models

Per @Luscha PR #1477 : Nullable fields generated with json omit empty tag

Nullable fields should be generated with json omit empty tag.
There are cases where the graphql server accepts only one of the field to perform a mutation (an example could be performing a mutation on an union field which expects only one of the type reference set).

This is the case working in particular with Dgraph, which expects a strict format for inputs.

Per @joshpauline in #2325

There is already an open pr for this feature however it had merge conflicts so I would like to fix those and merge this. This feature helps a lot when you are using the generated structs with external services. e.g Saving data to a DB

I have rebased that PR and regenerated it

[StevenACoffman]

Looks like rebasing isn't enough, and it requires a bit of cleanup, but I'm not able to give it attention right now. Help wanted.

[coveralls]

Coverage: 75.407% (-0.03%) from 75.441% when pulling 590f62b on josh into acbae6f on master.

[koenpunt]

I believe omitempty will also leave out fields which are explicitly set to nil, which isn't correct; in graphql there's a difference between sending in null and omitting a field.

[StevenACoffman]

Hmmm... I won't get a chance to look at this soon, so if you can make a PR, I would appreciate it.

[adomaskizogian]

I believe omitempty will also leave out fields which are explicitly set to nil, which isn't correct; in graphql there's a difference between sending in null and omitting a field.

Having the same question. Has anyone experienced any issues on live systems?

Update: If anyone stumbles upon this, know that at the moment I wrote this, gqlgen used a custom built gql marshaler that is totally unrelated to encoding/json

type Marshaler interface {
    MarshalGQL(w io.Writer)
}