Wrong TypeScript type when using `encore:"optional"` without `omitempty`

[melkstam]

When using the encore:"optional" and generating a client, the field is marked as field?: type. However, this typing is only correct when omitempty is included in the json tag.

Sometimes you actually don't want to add an omitempty, for example if you want to convey that there is a field here, but it just has no value.

---

For example, take this simple endpoint:

package hello

import (
	"context"
)

type HelloResponse struct {
	Foo *string `json:"foo" encore:"optional"`
	Bar *string `json:"bar,omitempty" encore:"optional"`
}

// encore:api public
func Hello(ctx context.Context) (*HelloResponse, error) {
	return &HelloResponse{
		Foo: nil,
		Bar: nil,
	}, nil
}

When generating a TypeScript client, the type of HelloResponse is

export interface HelloResponse {
    foo?: string
    bar?: string
}

However, when actually calling the endpoint and parsing it in JavaScript, the response you get is

{
    "foo": null
}

which actually matches to the type

export interface HelloResponse {
    foo: string | null
    bar?: string
}