Editor code: tidy up BOOLs,NULLs and overrides pt2.

A few 'typedefs' replaced by 'using's
This shouldn't have any functional changes at all, just c++17 modernization
It's a part 2 of a split #2847

Signed-off-by: Nemerle <nemerle5+git@gmail.com>

Code/Editor/Plugins/ComponentEntityEditorPlugin/SandboxIntegration.cpp

@@ -1815,7 +1815,7 @@ void SandboxIntegrationManager::ContextMenu_PushEntitiesToSlice(AzToolsFramework
     (void)targetAncestorId;
     (void)affectEntireHierarchy;
 
-    AZ::SerializeContext* serializeContext = NULL;
+    AZ::SerializeContext* serializeContext = nullptr;
     EBUS_EVENT_RESULT(serializeContext, AZ::ComponentApplicationBus, GetSerializeContext);
     AZ_Assert(serializeContext, "No serialize context");
 

Code/Editor/Plugins/ComponentEntityEditorPlugin/UI/ComponentPalette/ComponentPaletteWindow.cpp

@@ -42,7 +42,7 @@ void ComponentPaletteWindow::Init()
     layout->setContentsMargins(0, 0, 0, 0);
     layout->setSpacing(0);
 
-    QHBoxLayout* gridLayout = new QHBoxLayout(NULL);
+    QHBoxLayout* gridLayout = new QHBoxLayout(nullptr);
     gridLayout->setSizeConstraint(QLayout::SetMaximumSize);
     gridLayout->setContentsMargins(0, 0, 0, 0);
     gridLayout->setSpacing(0);