Fixed Crash when the scope string is NSNull

Author: Gernot

NXOAuth2Client.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name     = 'NXOAuth2Client'
-  s.version  = '1.2.3'
+  s.version  = '1.2.4'
   s.license  = {
     :type => 'BSD',
     :text => <<-LICENSETEXT

Sources/OAuth2Client/NXOAuth2AccessToken.m

@@ -62,7 +62,7 @@ + (id)tokenWithResponseBody:(NSString *)theResponseBody tokenType:(NSString *)to
     }
 
     NSSet *scope = nil;
-    if (scopeString) {
+    if (scopeString && ![scopeString isEqual:[NSNull null]]) {
         scope = [NSSet setWithArray:[scopeString componentsSeparatedByCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]]];
     }