LibJS: Soothe gcc about printf-%s on (non-)nullptr

Author: https://github.com/BenWiederhake
Commit: 801058e514
Pull-request: https://github.com/SerenityOS/serenity/pull/2882
Reviewed-by: https://github.com/alimpfard

Libraries/LibJS/AST.cpp

@@ -930,6 +930,7 @@ void ClassDeclaration::dump(int indent) const
 void ClassExpression::dump(int indent) const
 {
     print_indent(indent);
+    ASSERT(m_name.characters());
     printf("ClassExpression: \"%s\"\n", m_name.characters());
 
     print_indent(indent);