Editor code: tidy up BOOLs,NULLs and overrides pt1.

A few 'typedefs' replaced by 'using's
This shouldn't have any functional changes at all, just c++17 modernization
It's a part 1 of a split #2847

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

Code/Editor/Dialogs/ErrorsDlg.cpp

@@ -20,7 +20,7 @@ AZ_PUSH_DISABLE_DLL_EXPORT_MEMBER_WARNING
 AZ_POP_DISABLE_DLL_EXPORT_MEMBER_WARNING
 
 
-CErrorsDlg::CErrorsDlg(QWidget* pParent /*=NULL*/)
+CErrorsDlg::CErrorsDlg(QWidget* pParent /*=nullptr*/)
     : QDialog(pParent)
     , ui(new Ui::CErrorsDlg)
 {

Code/Editor/Dialogs/PythonScriptsDialog.cpp

@@ -159,7 +159,7 @@ void CPythonScriptsDialog::OnExecute()
     QList<QStandardItem*> selectedItems = ui->treeView->GetSelectedItems();
     QStandardItem* selectedItem = selectedItems.empty() ? nullptr : selectedItems.first();
 
-    if (selectedItem == NULL)
+    if (selectedItem == nullptr)
     {
         return;
     }