Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning.

Test: m
Bug: 68236239
Change-Id: I41ea089f2e7e86b432e8edc540600080097f44f3
diff --git a/ast_java.h b/ast_java.h
index 4b83465..935fc56 100644
--- a/ast_java.h
+++ b/ast_java.h
@@ -250,7 +250,7 @@
 
   explicit VariableDeclaration(Variable* lvalue);
   VariableDeclaration(Variable* lvalue, Expression* rvalue,
-                      const Type* cast = NULL);
+                      const Type* cast = nullptr);
   virtual ~VariableDeclaration() = default;
   void Write(CodeWriter* to) const override;
 };