Change org.junit.Assert.assertNull to kotlin.test.assertNull.

Change-Id: I60083b0b9a1dcaed84d957b49c91e5163d5659d6

feature/search/src/test/kotlin/com/google/samples/apps/nowinandroid/feature/search/SearchViewModelTest.kt

@@ -37,12 +37,12 @@ import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.test.UnconfinedTestDispatcher
 import kotlinx.coroutines.test.runTest
-import org.junit.Assert.assertNull
 import org.junit.Before
 import org.junit.Rule
 import org.junit.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertIs
+import kotlin.test.assertNull
 
 /**
  * To learn more about how this test handles Flows created with stateIn, see