JS backend: fix compiler crash on empty then if block

#EA-68941 fixed

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
+}