JS backend: fixed wrong behavior when using nullable type in string template.

(cherry picked from commit 3a16e95)

js/js.translator/testFiles/expression/stringTemplates/cases/stringValues.kt

@@ -7,5 +7,8 @@ fun box(): Boolean {
 
     if (message != "a = abc, b = def") return false
 
+    val v1 = null
+    if ("returns null null" != "returns $v1 ${null}") return false
+
     return true
 }