[clang] Use {} instead of std::nullopt to initialize empty ArrayRef (#109399)

Follow up to #109133.

Author: jayfoad

clang/include/clang/AST/CommentSema.h

@@ -80,7 +80,7 @@ class Sema {
   ArrayRef<T> copyArray(ArrayRef<T> Source) {
     if (!Source.empty())
       return Source.copy(Allocator);
-    return std::nullopt;
+    return {};
   }
 
   ParagraphComment *actOnParagraphComment(

clang/include/clang/AST/DeclFriend.h

@@ -115,7 +115,7 @@ class FriendDecl final
   static FriendDecl *
   Create(ASTContext &C, DeclContext *DC, SourceLocation L, FriendUnion Friend_,
          SourceLocation FriendL, SourceLocation EllipsisLoc = {},
-         ArrayRef<TemplateParameterList *> FriendTypeTPLists = std::nullopt);
+         ArrayRef<TemplateParameterList *> FriendTypeTPLists = {});
   static FriendDecl *CreateDeserialized(ASTContext &C, GlobalDeclID ID,
                                         unsigned FriendTypeNumTPLists);