JS: when both clauses of if become empty during optimization, remove if entirely. Make condition and then clause of JsIf non-nullable. Fix #KT-13912

js/js.translator/src/org/jetbrains/kotlin/js/translate/expression/ExpressionVisitor.java

@@ -285,6 +285,9 @@ public final class ExpressionVisitor extends TranslatorVisitor<JsNode> {
                 return new JsConditional(testExpression, jsThenExpression, jsElseExpression).source(expression);
             }
         }
+        if (thenStatement == null) {
+            thenStatement = JsEmpty.INSTANCE;
+        }
         JsIf ifStatement = new JsIf(testExpression, thenStatement, elseStatement);
         return ifStatement.source(expression);
     }

js/js.translator/testData/js-optimizer/redundant-label-removal/emptyIfConditionPreserved.optimized.js

@@ -0,0 +1,17 @@
+var global = "";
+
+function id(value) {
+    global += value + ";";
+    return value;
+}
+
+function test(x) {
+    id(x);
+    id(x + 1);
+}
+
+function box() {
+    test(23);
+    if (global != "23;24;") return "fail";
+    return "OK";
+}

js/js.translator/testData/js-optimizer/redundant-label-removal/emptyIfConditionPreserved.original.js

@@ -0,0 +1,23 @@
+var global = "";
+
+function id(value) {
+    global += value + ";";
+    return value;
+}
+
+function test(x) {
+    $outer: {
+        if (id(x) + id(x + 1) > 0) {
+            break $outer;
+        }
+        else {
+            break $outer;
+        }
+    }
+}
+
+function box() {
+    test(23);
+    if (global != "23;24;") return "fail";
+    return "OK";
+}

js/js.translator/testData/js-optimizer/redundant-label-removal/ifWithEmptyThenAndNoElse.optimized.js

@@ -0,0 +1,7 @@
+function test(x) {
+    return "OK";
+}
+
+function box() {
+    return test(23);
+}

js/js.translator/testData/js-optimizer/redundant-label-removal/ifWithEmptyThenAndNoElse.original.js

@@ -0,0 +1,12 @@
+function test(x) {
+    $outer: {
+        if (x > 10) {
+            break $outer;
+        }
+    }
+    return "OK";
+}
+
+function box() {
+    return test(23);
+}

js/js.translator/testData/sourcemap/cases/emptyIfInsideInlineLambda.kt

@@ -0,0 +1,19 @@
+package foo
+
+fun test1(): String {
+    run {
+        if (false) {
+        }
+    }
+    return "O"
+}
+
+fun test2(): String {
+    1.let {
+        if (false) {
+        }
+    }
+    return "K"
+}
+
+fun box() = test1() + test2()