Allow nullable types in Flow's firstOrNull extension

This closes Kotlin#2229

Author: ansman

kotlinx-coroutines-core/common/src/flow/terminal/Reduce.kt

@@ -124,7 +124,7 @@ public suspend fun <T> Flow<T>.first(predicate: suspend (T) -> Boolean): T {
  * The terminal operator that returns the first element emitted by the flow and then cancels flow's collection.
  * Returns `null` if the flow was empty.
  */
-public suspend fun <T : Any> Flow<T>.firstOrNull(): T? {
+public suspend fun <T> Flow<T>.firstOrNull(): T? {
     var result: T? = null
     try {
         collect { value ->
@@ -138,10 +138,10 @@ public suspend fun <T : Any> Flow<T>.firstOrNull(): T? {
 }
 
 /**
- *  The terminal operator that returns the first element emitted by the flow matching the given [predicate] and then cancels flow's collection.
+ * The terminal operator that returns the first element emitted by the flow matching the given [predicate] and then cancels flow's collection.
  * Returns `null` if the flow did not contain an element matching the [predicate].
  */
-public suspend fun <T : Any> Flow<T>.firstOrNull(predicate: suspend (T) -> Boolean): T? {
+public suspend fun <T> Flow<T>.firstOrNull(predicate: suspend (T) -> Boolean): T? {
     var result: T? = null
     try {
         collect { value ->

kotlinx-coroutines-core/common/test/flow/terminal/FirstTest.kt

@@ -127,6 +127,12 @@ class FirstTest : TestBase() {
         assertNull(emptyFlow<Int>().firstOrNull { true })
     }
 
+    @Test
+    fun testFirstOrNullWithNullElement() = runTest {
+        assertNull(flowOf<String?>(null).firstOrNull())
+        assertNull(flowOf<String?>(null).firstOrNull { true })
+    }
+
     @Test
     fun testFirstOrNullWhenErrorCancelsUpstream() = runTest {
         val latch = Channel<Unit>()