Update Grants.kt make Scope optional (#210)

Make Scope optional like in the API call

https://nylas.atlassian.net/browse/TW-2566

# License

I confirm that this contribution is made under the terms of the MIT
license and that I have the authority necessary to make this
contribution on behalf of its copyright owner.

src/main/kotlin/com/nylas/models/Grant.kt

@@ -20,7 +20,7 @@ data class Grant(
    * Scopes specified for the grant.
    */
   @Json(name = "scope")
-  val scope: List<String>,
+  val scope: List<String>? = null,
   /**
    * Unix timestamp when the grant was created.
    */