Add validation for empty identifier in mapActorAlias

Ensured that the `identifier` parameter in `mapActorAlias()` is not an
empty string.  Added a regression test to verify this validation.

#753 (comment)

Assisted-by: Gemini CLI:gemini-3-flash-preview
Assisted-by: Codex:gpt-5.5

Author: dahlia

packages/fedify/src/federation/builder.test.ts

@@ -53,6 +53,14 @@ test("FederationBuilder", async (t) => {
         RouterError,
         'Actor alias path "/actor" conflicts with existing route "actorAlias:instance".',
       );
+      assertThrows(
+        () =>
+          createFederationBuilder<string>()
+            .setActorDispatcher("/users/{identifier}", actorDispatcher)
+            .mapActorAlias("/actor", ""),
+        RouterError,
+        "Identifier cannot be empty.",
+      );
       builder.setActorDispatcher("/users/{identifier}", actorDispatcher)
         .mapActorAlias("/actor", "instance");
 

packages/fedify/src/federation/builder.ts

@@ -525,6 +525,9 @@ export class FederationBuilderImpl<TContextData>
         return setters;
       },
       mapActorAlias: (path: `/${string}`, identifier: string) => {
+        if (identifier === "") {
+          throw new RouterError("Identifier cannot be empty.");
+        }
         if (this.router.has(`${ACTOR_ALIAS_PREFIX}${identifier}`)) {
           throw new RouterError(
             `Actor alias for "${identifier}" already set.`,