Add new type alias StringNullWhenEmpty for nullable string values

[manicminer]

• This enables zeroing field values that don't accept blank strings
• Also tidy up models and split value types into another file
• Support PersonalMicrosoftAccount for application signInAudience

Example usage

displayName := "test group no description"
description := msgraph.StringNullWhenEmpty("")
mailEnabled := false
securityEnabled := true

props := msgraph.Group{
	DisplayName:     &displayName,
	Description:     &description,
	MailEnabled:     &mailEnabled,
	SecurityEnabled: &securityEnabled,
}

group, _, err := client.Create(ctx, props)

Generates the following request body:

{
  "description": null,
  "displayName": "test group no description",
  "mailEnabled": false,
  "securityEnabled": true,
}