added Test for quieter if volume is 0

src/test/java/device/radioPlayer/RadioPlayerTest.java

@@ -27,16 +27,20 @@ class RadioPlayerTest {
         RadioPlayer rp1 = new RadioPlayer();
         RadioPlayer rp2 = new RadioPlayer();
         RadioPlayer rp3 = new RadioPlayer();
+        RadioPlayer rp4 = new RadioPlayer();
         rp1.setLautstaerke(0);
         rp1.louder();
         rp2.setLautstaerke(100);
         rp2.louder();
         rp3.setLautstaerke(1);
         rp3.quieter();
+        rp4.setLautstaerke(0);
+        rp4.quieter();
         return Stream.of(
                 Arguments.of("Test for one step louder", rp1, 1),
                 Arguments.of("Test for max louder", rp2, 100),
-                Arguments.of("Test for one step quieter", rp3, 0)
+                Arguments.of("Test for one step quieter", rp3, 0),
+                Arguments.of("Test for min quieter", rp4, 0)
         );
     }