Add `null` to the list of reserved words.

The previous change claimed to do this in addition to `true` and `false`, but it was not actually in the code.

To reiterate the text from the earlier change, now actually reflected entirely in the code:

> The identifiers `true`, `false`, and `null` are effectively reserved words in Java, although for some reason they are listed separately from the "keywords" in the Java Language Specification.
>
> This doesn't matter for regular fields, because a proto field called `true` will be accessed with `getTrue` and `setTrue`. But for extensions, the generated Java code will have a public static field whose name is the same as the name of the extension field, with `_` appended if the name is a reserved word. Previously there was no `_` for `true` etc, so the generated code would not compile.

This change should not affect any existing client code in Java. If someone had tried to use an extension called `true` (etc), they would have found that the generated proto code did not compile. Now it is possible to reference such an extension as `true_`.

PiperOrigin-RevId: 632174190

src/google/protobuf/compiler/java/names.cc

@@ -39,17 +39,17 @@ const char* DefaultPackage(Options options) {
 bool IsReservedName(absl::string_view name) {
   static const auto& kReservedNames =
       *new absl::flat_hash_set<absl::string_view>({
-          "abstract", "assert",     "boolean",      "break",     "byte",
-          "case",     "catch",      "char",         "class",     "const",
-          "continue", "default",    "do",           "double",    "else",
-          "enum",     "extends",    "false",        "final",     "finally",
-          "float",    "for",        "goto",         "if",        "implements",
-          "import",   "instanceof", "int",          "interface", "long",
-          "native",   "new",        "package",      "private",   "protected",
-          "public",   "return",     "short",        "static",    "strictfp",
-          "super",    "switch",     "synchronized", "this",      "throw",
-          "throws",   "transient",  "true",         "try",       "void",
-          "volatile", "while",
+          "abstract",  "assert",     "boolean",   "break",        "byte",
+          "case",      "catch",      "char",      "class",        "const",
+          "continue",  "default",    "do",        "double",       "else",
+          "enum",      "extends",    "false",     "final",        "finally",
+          "float",     "for",        "goto",      "if",           "implements",
+          "import",    "instanceof", "int",       "interface",    "long",
+          "native",    "new",        "null",      "package",      "private",
+          "protected", "public",     "return",    "short",        "static",
+          "strictfp",  "super",      "switch",    "synchronized", "this",
+          "throw",     "throws",     "transient", "true",         "try",
+          "void",      "volatile",   "while",
       });
   return kReservedNames.contains(name);
 }