Use INativeObject.GetHandle in a few places to simplify code.

Since INativeObject.GetHandle is an extension method that can detect a null
'this' value and returns IntPtr.Zero.

src/Network/NWConnection.cs

@@ -387,7 +387,7 @@ namespace Network {
 		unsafe void LowLevelSend (IntPtr handle, DispatchData buffer, IntPtr contentContext, bool isComplete, void *callback)
 		{
 			nw_connection_send (handle: GetCheckedHandle (),
-					    dispatchData: buffer == null ? IntPtr.Zero : buffer.Handle,
+					    dispatchData: buffer.GetHandle (),
 					    contentContext: contentContext,
 					    isComplete: isComplete,
 					    callback: callback);

src/Network/NWContentContext.cs

@@ -121,7 +121,7 @@ namespace Network {
 				return new NWContentContext (h, owns: true);
 			}
 			set {
-				nw_content_context_set_antecedent (GetCheckedHandle (), value == null ? IntPtr.Zero : value.Handle);
+				nw_content_context_set_antecedent (GetCheckedHandle (), value.GetHandle ());
 			}
 		}
 

src/Network/NWListener.cs

@@ -214,7 +214,7 @@ namespace Network {
 
 		public void SetAdvertiseDescriptor (NWAdvertiseDescriptor descriptor)
 		{
-			nw_listener_set_advertise_descriptor (GetCheckedHandle (), descriptor == null ? IntPtr.Zero : descriptor.handle);
+			nw_listener_set_advertise_descriptor (GetCheckedHandle (), descriptor.GetHandle ());
 		}
 	}
 }

src/Network/NWParameters.cs

@@ -271,7 +271,7 @@ namespace Network {
 		[TV (12,0), Mac (10,14), iOS (12,0)]
 		public void RequireInterface (NWInterface iface)
 		{
-			nw_parameters_require_interface (GetCheckedHandle (), iface == null ? IntPtr.Zero : iface.handle);
+			nw_parameters_require_interface (GetCheckedHandle (), iface.GetHandle ());
 		}
 
 		[TV (12,0), Mac (10,14), iOS (12,0)]
@@ -491,7 +491,7 @@ namespace Network {
 			}
 
 			set {
-				nw_parameters_set_local_endpoint (GetCheckedHandle (), value == null ? IntPtr.Zero : value.handle);
+				nw_parameters_set_local_endpoint (GetCheckedHandle (), value.GetHandle ());
 			}
 		}
 	}

src/Network/NWProtocolStack.cs

@@ -87,7 +87,7 @@ namespace Network {
 		[TV (12, 0), Mac (10, 14), iOS (12, 0)]
 		public NWProtocolOptions TransportProtocol {
 			get => new NWProtocolOptions (nw_protocol_stack_copy_transport_protocol (GetCheckedHandle ()), owns: true);
-			set => nw_protocol_stack_set_transport_protocol (GetCheckedHandle (), value.Handle);
+			set => nw_protocol_stack_set_transport_protocol (GetCheckedHandle (), value.GetHandle ());
 		}
 
 		[TV (12, 0), Mac (10, 14), iOS (12, 0)]