Null-terminate Collection.toArray destination only in JVM

In other platforms the elements following the collection elements
should not be changed.

As a part of efforts to stabilize Native stdlib.

js/js.translator/testData/box/regression/stdlibTestSnippets/abstractCollectionToArray.kt

@@ -24,7 +24,7 @@ fun abstractCollectionToArray() {
     assertTrue("toArray1" in coll.invocations || "toArray2" in coll.invocations)
 
     val arr2: Array<String> = coll.toArray(Array(coll.size + 1) { "" })
-    assertEquals(data + listOf(null), arr2.asList())
+    assertEquals(data + listOf(""), arr2.asList())
 }
 
 fun box(): String {