improve tests (#6505)

Author: korshaknn

libnavui-dropin/src/test/java/com/mapbox/navigation/dropin/navigationview/MapboxNavigationViewApiTest.kt

@@ -358,6 +358,9 @@ class MapboxNavigationViewApiTest {
         val player: MapboxVoiceInstructionsPlayer = mockk()
         every { audioGuidance.getCurrentVoiceInstructionsPlayer() } returns player
         assertEquals(player, sut.getCurrentVoiceInstructionsPlayer())
+
+        every { audioGuidance.getCurrentVoiceInstructionsPlayer() } returns null
+        assertNull(sut.getCurrentVoiceInstructionsPlayer())
     }
 
     private fun navigationRoute(vararg waypoints: Point): NavigationRoute {

libnavui-voice/src/test/java/com/mapbox/navigation/ui/voice/api/MapboxAudioGuidanceTest.kt

@@ -223,5 +223,8 @@ class MapboxAudioGuidanceTest {
         val player: MapboxVoiceInstructionsPlayer = mockk()
         every { mapboxAudioGuidanceServices.voiceInstructionsPlayer } returns player
         assertEquals(player, carAppAudioGuidance.getCurrentVoiceInstructionsPlayer())
+
+        every { mapboxAudioGuidanceServices.voiceInstructionsPlayer } returns null
+        assertNull(carAppAudioGuidance.getCurrentVoiceInstructionsPlayer())
     }
 }