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/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()