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
     }
 

libraries/stdlib/src/kotlin/text/StringBuilder.kt

@@ -77,9 +77,11 @@ expect class StringBuilder : Appendable, CharSequence {
 
     /**
      * 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")
-    fun append(value: String): StringBuilder
+    fun append(value: String?): StringBuilder
 
     /**
      * Returns the current capacity of this string builder.
@@ -200,11 +202,13 @@ expect class StringBuilder : Appendable, CharSequence {
     /**
      * 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)
-    fun insert(index: Int, value: String): StringBuilder
+    fun insert(index: Int, value: String?): StringBuilder
 
     /**
      *  Sets the length of this string builder to the specified [newLength].

libraries/stdlib/test/text/StringBuilderTest.kt

@@ -153,6 +153,9 @@ class StringBuilderTest {
         StringBuilder().let { sb ->
             repeat(times) { sb.append("foo") }
             assertEquals(expected, sb.toString())
+
+            sb.append(null as String?)
+            assertEquals(expected + "null", sb.toString())
         }
     }
 
@@ -384,6 +387,8 @@ class StringBuilderTest {
             assertEquals("_my insertTtT string test", sb.toString())
             sb.insert(25, "_!_")
             assertEquals("_my insertTtT string test_!_", sb.toString())
+            sb.insert(13, null as String?)
+            assertEquals("_my insertTtTnull string test_!_", sb.toString())
         }
     }