test(search): add empty-receivers edge case for RECEIVER sort

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

backend/src/test/java/org/raddatz/familienarchiv/service/DocumentServiceTest.java

@@ -1294,6 +1294,26 @@ class DocumentServiceTest {
         assertThat(result).extracting(Document::getTitle).containsExactly("Has Sender", "No Sender");
     }
 
+    // ─── searchDocuments — RECEIVER sort, empty receivers ───────────────────────
+
+    @Test
+    void searchDocuments_receiverSort_emptyReceiversSortsToEnd() {
+        Person alice = Person.builder().id(UUID.randomUUID()).firstName("Alice").lastName("Maier").build();
+        Document withReceiver = Document.builder().id(UUID.randomUUID()).title("Has Receiver")
+                .receivers(new HashSet<>(Set.of(alice))).build();
+        Document noReceivers = Document.builder().id(UUID.randomUUID()).title("No Receivers")
+                .receivers(new HashSet<>()).build();
+
+        when(documentRepository.findAll(any(org.springframework.data.jpa.domain.Specification.class)))
+                .thenReturn(List.of(noReceivers, withReceiver));
+
+        List<Document> result = documentService.searchDocuments(
+                null, null, null, null, null, null, null, null, DocumentSort.RECEIVER, "asc");
+
+        assertThat(result).extracting(Document::getTitle)
+                .containsExactly("Has Receiver", "No Receivers");
+    }
+
     @Test
     void searchDocuments_senderSort_nullLastNameSortsToEnd() {
         // Without fix: null lastName produces sort key "null Smith" which compares