Add NULL as reserved keyword cpp (OpenAPITools#17803)

* [cpp]Do not lowercase the reserved keywords for C++

(Will be necessary for adding NULL as a reserved keyword)

* [cpp]Add NULL as reserved keyword

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractCppCodegen.java

@@ -41,6 +41,7 @@
 import java.net.URL;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -61,7 +62,7 @@ public AbstractCppCodegen() {
         /*
          * Reserved words.  Override this with reserved words specific to your language
          */
-        setReservedWordsLowerCase(
+        reservedWords = new HashSet<>(
                 Arrays.asList(
                         "alignas",
                         "alignof",
@@ -112,6 +113,7 @@ public AbstractCppCodegen() {
                         "noexcept",
                         "not",
                         "not_eq",
+                        "NULL",
                         "nullptr",
                         "operator",
                         "or",