Fix overzealous assert.

Sometimes we set values to nil - e.g. when re-registering through
debugUI

// FREEBIE

Signal/src/ViewControllers/OWS2FAReminderViewController.swift

@@ -0,0 +1,9 @@
+//
+//  ReminderViewController.swift
+//  Signal
+//
+//  Created by Michael Kirk on 3/1/18.
+//  Copyright © 2018 Open Whisper Systems. All rights reserved.
+//
+
+import Foundation

SignalServiceKit/src/Storage/YapDatabaseConnection+OWS.m

@@ -117,7 +117,6 @@ NS_ASSUME_NONNULL_BEGIN
 
 - (void)setObject:(id)object forKey:(NSString *)key inCollection:(NSString *)collection
 {
-    OWSAssert(object);
     OWSAssert(key.length > 0);
     OWSAssert(collection.length > 0);