Small improvement for String.length (dotnet#9469)

* Simplify and improve perf of String.length

* Improve performance of String.map

* Revert "Improve performance of String.map"

* Turn String.length into a one-liner, fixes dotnet#9469 (comment)

Author: abelbraaksma

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