Fix module parser to accept undefined (???) body

The module parser required at least one real definition inside
its body, unlike every other scope parser which accepts ???.
This caused parse errors for `module X is { ??? }`.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: reid-spencer

language/shared/src/main/scala/com/ossuminc/riddl/language/parsing/ModuleParser.scala

@@ -34,7 +34,9 @@ private[parsing] trait ModuleParser {
 
   def module[u: P]: P[Module] = {
     P(
-      Index ~ Keywords.module ~/ identifier ~ is ~ open ~ moduleContents ~ close ~ withMetaData ~ Index
+      Index ~ Keywords.module ~/ identifier ~ is ~ open ~
+        (undefined(Seq.empty[ModuleContents]) | moduleContents) ~
+        close ~ withMetaData ~ Index
     )./.map { case (start, id, contents, descriptives, end) =>
       checkForDuplicateIncludes(contents)
       Module(at(start, end), id, contents.toContents, descriptives.toContents)