fix(backend): Update typing of Organization.slug (#5636)

Author: tmilewski

.changeset/solid-heads-turn.md

@@ -0,0 +1,5 @@
+---
+'@clerk/backend': patch
+---
+
+Update typing of Organization.slug

packages/backend/src/api/resources/Organization.ts

@@ -10,7 +10,7 @@ export class Organization {
   constructor(
     readonly id: string,
     readonly name: string,
-    readonly slug: string | null,
+    readonly slug: string,
     readonly imageUrl: string,
     readonly hasImage: boolean,
     readonly createdAt: number,