Add omitempty json tags for nullable fields

[0xfourzerofour]

There is already an open pr for this feature however it had merge conficts 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 an DB

[StevenACoffman]

Thanks for freshening this up! It looks like there are some lint and test problems with this PR.

Can you try:

go generate; cd _examples; go generate; cd ..
go test ./...; cd _examples; go test ./...