fix(test): use nullValue() matcher instead of doesNotExist() for null note field

doesNotExist() asserts the key is absent from the JSON object, but Jackson
serializes a null Optional<String> as {"note": null} — the key is present with
a null value. nullValue() correctly matches that case.

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

backend/src/test/java/org/raddatz/familienarchiv/geschichte/GeschichteControllerTest.java

@@ -24,6 +24,7 @@ import java.util.HashSet;
 import java.util.List;
 import java.util.UUID;
 
+import static org.hamcrest.CoreMatchers.nullValue;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyInt;
 import static org.mockito.ArgumentMatchers.eq;
@@ -283,7 +284,7 @@ class GeschichteControllerTest {
                         .contentType(MediaType.APPLICATION_JSON)
                         .content("{\"note\": null}"))
                 .andExpect(status().isOk())
-                .andExpect(jsonPath("$.note").doesNotExist());
+                .andExpect(jsonPath("$.note").value(nullValue()));
     }
 
     @Test