FIR resolve: add test for firstOrNull

compiler/fir/resolve/testData/resolve/stdlib/arrayFirstOrNull.kt

@@ -0,0 +1,11 @@
+interface G {
+    val a: Array<out G>
+}
+
+fun goo(g: G) {
+    val x = g.a.firstOrNullX()
+}
+
+public fun <T> Array<out T>.firstOrNullX(): T? {
+    return if (isEmpty()) null else this[0]
+}

compiler/fir/resolve/testData/resolve/stdlib/arrayFirstOrNull.txt

@@ -0,0 +1,20 @@
+FILE: arrayFirstOrNull.kt
+    public abstract interface G : R|kotlin/Any| {
+        public abstract val a: R|kotlin/Array<out G>|
+            public get(): R|kotlin/Array<out G>|
+
+    }
+    public final fun goo(g: R|G|): R|kotlin/Unit| {
+        lval x: R|G|? = R|<local>/g|.R|/G.a|.R|/firstOrNullX|<R|G|>()
+    }
+    public final fun <T> R|kotlin/Array<out T>|.firstOrNullX(): R|T|? {
+        ^firstOrNullX when () {
+            <Unresolved name: isEmpty>#() ->  {
+                Null(null)
+            }
+            else ->  {
+                this#.R|FakeOverride<kotlin/Array.get: R|T|>|(Int(0))
+            }
+        }
+
+    }

compiler/fir/resolve/tests/org/jetbrains/kotlin/fir/FirResolveTestCaseWithStdlibGenerated.java

@@ -29,6 +29,11 @@ public class FirResolveTestCaseWithStdlibGenerated extends AbstractFirResolveTes
         KotlinTestUtils.assertAllTestsPresentByMetadata(this.getClass(), new File("compiler/fir/resolve/testData/resolve/stdlib"), Pattern.compile("^([^.]+)\\.kt$"), TargetBackend.ANY, true);
     }
 
+    @TestMetadata("arrayFirstOrNull.kt")
+    public void testArrayFirstOrNull() throws Exception {
+        runTest("compiler/fir/resolve/testData/resolve/stdlib/arrayFirstOrNull.kt");
+    }
+
     @TestMetadata("concurrent.kt")
     public void testConcurrent() throws Exception {
         runTest("compiler/fir/resolve/testData/resolve/stdlib/concurrent.kt");