Fix code exchange response for scope value (#231)

# Description

Marking `scope` as a default value
https://nylas.atlassian.net/browse/TSDK-609

# License
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src/main/kotlin/com/nylas/models/CodeExchangeResponse.kt

@@ -25,7 +25,7 @@ data class CodeExchangeResponse(
    * List of scopes associated with this token.
    */
   @Json(name = "scope")
-  val scope: String,
+  val scope: String? = null,
   /**
    * Email address of the grant that is created.
    */