Fix System.ArgumentNullException in notificationhub implementation

StackTrace from Visual Studio 2013:

04-11 10:31:29.039 E/mono    ( 2988): Unhandled Exception:
04-11 10:31:29.039 E/mono    ( 2988):   at (wrapper delegate-invoke) <Module>:invoke_string[]__this___Registration (ByteSmith.WindowsAzure.Messagi04-11 10:31:29.039 E/mono    ( 2988): System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.ArgumentNullException: Argument cannot be null.
04-11 10:31:29.039 E/mono    ( 2988): Parameter name: source
04-11 10:31:29.039 E/mono    ( 2988):   at System.Linq.Check.Source (System.Object source) [0x00000] in <filename unknown>:0
04-11 10:31:29.039 E/mono    ( 2988):   at System.Linq.Enumerable.ToArray[String] (IEnumerable`1 source) [0x00000] in <filename unknown>:0
04-11 10:31:29.039 E/mono    ( 2988):   at ByteSmith.WindowsAzure.Messaging.Registration.get_TagsSurrogate () [0x00001] in \ByteSmith.WindowsAzure.Messaging\Registration.cs:34
04-11 10:31:29.039 E/mono    ( 2988):   at (wrapper delegate-invoke) <Module>:invoke_string[]__this___Registration (ByteSmith.WindowsAzure.Messaging.Registration)
04-11 10:31:29.039 E/mono    ( 2988):   at System.Reflection.MonoProperty.GetterAdapterFrame[Registration,String[]] (System.Reflection.Getter`2 getter, System.Object obj) [0x00000] in <filename unknown>:0
04-11 10:31:29.039 E/mono    ( 2988):   at System.Reflection.MonoProperty.GetValue (System.Object obj, System.Object[] index) [0x00000] in <filename unknown>:0
04-11 10:31:29.039 E/mono    ( 2988):
04-11 10:31:29.072 E/mono-rt ( 2988): Stacktrace:
04-11 10:31:29.072 E/mono-rt ( 2988):
04-11 10:31:29.072 E/mono-rt ( 2988):   at <unknown> <0xffffffff>