JS: fix compiler crash when optimizing JS AST with empty do..while statement. See KT-15513

compiler/testData/codegen/box/controlStructures/breakContinueInExpressions/breakFromOuter.kt

@@ -1,6 +1,3 @@
-// TODO: muted automatically, investigate should it be ran for JS or not
-// IGNORE_BACKEND: JS
-
 fun box(): String {
     OUTER@while (true) {
         var x = ""

js/js.inliner/src/org/jetbrains/kotlin/js/inline/clean/DoWhileGuardElimination.kt

@@ -62,7 +62,7 @@ internal class DoWhileGuardElimination(private val root: JsStatement) {
                 val body = x.body
                 val guard = when (body) {
                     is JsBlock -> {
-                        val firstStatement = body.statements.first()
+                        val firstStatement = body.statements.firstOrNull()
                         if (firstStatement is JsLabel && body.statements.size == 1) {
                             firstStatement
                         }

js/js.tests/test/org/jetbrains/kotlin/js/test/optimizer/DoWhileGuardEliminationTest.kt

@@ -24,4 +24,6 @@ class DoWhileGuardEliminationTest : BasicOptimizerTest("do-while-guard-eliminati
     @Test fun innerContinue() = box()
 
     @Test fun innerBreakInLoopWithoutLabel() = box()
+
+    @Test fun emptyDoWhile() = box()
 }

js/js.tests/test/org/jetbrains/kotlin/js/test/semantics/JsCodegenBoxTestGenerated.java

@@ -5033,13 +5033,7 @@ public class JsCodegenBoxTestGenerated extends AbstractJsCodegenBoxTest {
             @TestMetadata("breakFromOuter.kt")
             public void testBreakFromOuter() throws Exception {
                 String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/box/controlStructures/breakContinueInExpressions/breakFromOuter.kt");
-                try {
-                    doTest(fileName);
-                }
-                catch (Throwable ignore) {
-                    return;
-                }
-                throw new AssertionError("Looks like this test can be unmuted. Remove IGNORE_BACKEND directive for that.");
+                doTest(fileName);
             }
 
             @TestMetadata("breakInDoWhile.kt")

js/js.translator/testData/js-optimizer/do-while-guard-elimination/emptyDoWhile.optimized.js

@@ -0,0 +1,5 @@
+function box() {
+    do {
+    } while (false);
+    return "OK"
+}

js/js.translator/testData/js-optimizer/do-while-guard-elimination/emptyDoWhile.original.js

@@ -0,0 +1,5 @@
+function box() {
+    do {
+    } while (false);
+    return "OK"
+}