[BUG] - Startup script updates do not omit empty values

[optik-aper]

Describe the bug
Updates use the same struct as create and therefore do not allow nil values to be passed in or omitted when the JSON body is marshalled for the request.

To Reproduce
Steps to reproduce the behavior:

• Create a script
• Modify all but one value in the script

	scriptID := "b44d064c-e437-4360-adf7-570501c2577e"
	req := &govultr.StartupScriptReq{
		Name:   "new name2",
		Script: "ZWNobyAnaGVsbG8gd29ybGQnCg==",
	}

	if err := client.StartupScript.Update(ctx, scriptID, req); err != nil {
		fmt.Println(err)
		panic(err.Error())
	}

• Note how Type is wiped out after this update since it is not provided

{
        "id": "b44d064c-e437-4360-adf7-570501c2577e",
        "date_created": "2024-01-25T23:08:23+00:00",
        "date_modified": "2024-01-25T23:20:37+00:00",
        "name": "new name2",
        "type": "",
        "script": "ZWNobyAnaGVsbG8gd29ybGQnCg=="
}

Expected behavior
Partial updates should be supported per the API documentation

[optik-aper]

Fixed in #299