Avoid mis-detecting NULL as part of other identifiers

Tighten the search for NULL in the code style check to avoid flagging
the use of MONITOR_DEFAULTTONULL (Win32 API constant) as violation.

.github/workflows/code_checks.yml

@@ -88,7 +88,7 @@ jobs:
             git fetch --depth=1 origin master
             if git diff origin/master \
                ':**.h' ':**.cpp' \
-               | grep -E '^\+.*(wxOVERRIDE|wxNOEXCEPT|[^"_@]NULL[^"_])'; then
+               | grep -E '^\+.*(wxOVERRIDE|wxNOEXCEPT|[^"_@A-Za-z0-9]NULL[^"_A-Za-z0-9])'; then
               echo "::error ::Please use C++11 equivalents of the deprecated macros in the new code."
               exit 1
             fi