Make ViewController nullable param in reauthorizeDataAccess (#2477)

Summary:
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## Pull Request Details

👋  hello from the [react-native-fbsdk-next](https://github.com/thebergamo/react-native-fbsdk-next) maintainer 😄

I would like the `reauthorizeDataAccess` ViewController param to conform to the nullability documented for it, and acceptable to the methods it passes the param to

Pull Request resolved: #2477

Test Plan:
Untested (not entirely true, we've been [actively sending nil through this API](https://github.com/thebergamo/react-native-fbsdk-next/pull/204/files#diff-17f183fc0de77f22952fc0fddd688c62adde2533c3dc32798d9f836390f08454R110) since [our first implementation using the API](thebergamo/react-native-fbsdk-next#204), and it's worked fine)

Fixes #2476

Differential Revision: D67113142

Pulled By: yuriy-tolstoguzov

fbshipit-source-id: 5fdc24b84cb2e10caceb928eb6df123452a0e66e

FBSDKLoginKit/FBSDKLoginKit/LoginManager.swift

@@ -303,7 +303,7 @@ public final class LoginManager: NSObject {
    */
   @objc(reauthorizeDataAccess:handler:)
   public func reauthorizeDataAccess(
-    from viewController: UIViewController,
+    from viewController: UIViewController?,
     handler: @escaping LoginManagerLoginResultBlock
   ) {
     guard