Make consistent parameter nullability with appendLine

libraries/stdlib/js/src/kotlin/text/StringBuilderJs.kt

@@ -116,10 +116,12 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
 
     /**
      * Appends the specified string [value] to this string builder and returns this instance.
+     *
+     * If [value] is `null`, then the four characters `"null"` are appended.
      */
     @SinceKotlin("1.3")
-    actual fun append(value: String): StringBuilder {
-        this.string += value
+    actual fun append(value: String?): StringBuilder {
+        this.string += value ?: "null"
         return this
     }
 
@@ -280,14 +282,17 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
     /**
      * Inserts the string [value] into this string builder at the specified [index] and returns this instance.
      *
+     * If [value] is `null`, then the four characters `"null"` are inserted.
+     *
      * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
      */
     @SinceKotlin("1.4")
     @WasExperimental(ExperimentalStdlibApi::class)
-    actual fun insert(index: Int, value: String): StringBuilder {
+    actual fun insert(index: Int, value: String?): StringBuilder {
         AbstractList.checkPositionIndex(index, length)
 
-        this.string = this.string.substring(0, index) + value + this.string.substring(index)
+        val toInsert = value ?: "null"
+        this.string = this.string.substring(0, index) + toInsert + this.string.substring(index)
         return this
     }