Merge pull request #9475 from dotnet/merges/master-to-release/dev16.7

Merge master to release/dev16.7

Author: KevinRansom

src/fsharp/FSharp.Core/string.fs

@@ -103,8 +103,4 @@ namespace Microsoft.FSharp.Core
                 check 0  
 
         [<CompiledName("Length")>]
-        let length (str:string) =
-            if String.IsNullOrEmpty str then
-                0
-            else
-                str.Length
+        let length (str:string) = if isNull str then 0 else str.Length