search flight should not be null

src/test/java/hs/fulda/de/ci/exam/project/AdminTest.java

@@ -110,4 +110,12 @@ public class AdminTest {
         assertEquals(admin.searchFlights("1"), flight1.toString());
     }
 
+    @Test
+    public void test_searchFlightThrowsExceptionWhenBlank(){
+        when(flightRepository.findFlightByFlightNumber(any(String.class))).thenReturn("");
+        assertThrows(RuntimeException.class, () -> {
+            admin.searchFlights("1");
+        });
+    }
+
 }