SharedPtr: NULL -> nullptr

ARMmbed/mbed-os#12048 will require that
`SharedPtr` users use `nullptr` rather than `NULL` or `0`, which
will no longer compile.

Update example in code.

Change can be made to docs whenever, as `nullptr` form works now.

Author: kjbracey

docs/api/platform/Callback.md

@@ -14,7 +14,7 @@ void run_timer_event(Callback<void(float)> on_timer) {
 }
 ```
 
-The only thing to watch out for is that the Callback type has a null Callback, just like a null function pointer. Uninitialized callbacks are null and assert if you call them. If you want a call to always succeed, you need to check if it is null first.
+The only thing to watch out for is that the Callback type has an empty Callback, just like a null function pointer. Default initialized callbacks are empty and assert if you call them. If a callback may be empty, you need to check if it is empty before calling it.
 
 ``` c++
 void run_timer_event(Callback<void(float)> on_timer) {

docs/api/platform/SharedPtr.md

@@ -27,8 +27,8 @@ void test() {
     // Increase reference count
     SharedPtr<MyStruct> ptr2( ptr );
 
-    ptr = NULL; // Reference to the struct instance is still held by ptr2
+    ptr = nullptr; // Reference to the struct instance is still held by ptr2
 
-    ptr2 = NULL; // The raw pointer is freed
+    ptr2 = nullptr; // The raw pointer is freed
 }
 ```