JS backend: fix compiler crash on empty then if block

#EA-68941 fixed

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

@@ -41,4 +41,8 @@ public class IfExpressionTest extends AbstractExpressionTest {
     public void testNestedIf() throws Exception {
         checkFooBoxIsOk();
     }
+
+    public void testWithEmptyBlocks() throws Exception {
+        checkFooBoxIsOk();
+    }
 }

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

@@ -215,12 +215,12 @@ public final class ExpressionVisitor extends TranslatorVisitor<JsNode> {
         boolean isKotlinExpression = BindingContextUtilPackage.isUsedAsExpression(expression, context.bindingContext());
 
         JetExpression thenExpression = expression.getThen();
-        assert thenExpression != null : "then expression should not be null: " + expression.getText();
         JetExpression elseExpression = expression.getElse();
 
-        JsStatement thenStatement = Translation.translateAsStatementAndMergeInBlockIfNeeded(thenExpression, context);
-        JsStatement elseStatement = (elseExpression != null) ? Translation.translateAsStatementAndMergeInBlockIfNeeded(elseExpression,
-                                                                                                                       context) : null;
+        JsStatement thenStatement =
+                thenExpression != null ? Translation.translateAsStatementAndMergeInBlockIfNeeded(thenExpression, context) : null;
+        JsStatement elseStatement =
+                elseExpression != null ? Translation.translateAsStatementAndMergeInBlockIfNeeded(elseExpression, context) : null;
 
         if (isKotlinExpression) {
             JsExpression jsThenExpression = JsAstUtils.extractExpressionFromStatement(thenStatement);

js/js.translator/testData/expression/if/cases/withEmptyBlocks.kt

@@ -0,0 +1,8 @@
+package foo
+
+fun box(): String {
+    var r = ""
+    if (r != "") else r += "O"
+    if (r == "O") r += "K" else;
+    return r
+}