Fix nullable

Unosquare.RaspberryIO.Abstractions/Native/HardwareException.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 using System;
 using System.Runtime.InteropServices;
 

Unosquare.RaspberryIO.Abstractions/Native/Standard.cs

@@ -16,7 +16,7 @@ namespace Unosquare.RaspberryIO.Abstractions.Native {
     /// </summary>
     /// <param name="error">The error.</param>
     /// <returns>The error string.</returns>
-    public static String? Strerror(Int32 error) {
+    public static String Strerror(Int32 error) {
       if(Type.GetType("Mono.Runtime") == null) {
         return Marshal.PtrToStringAnsi(StrError(error));
       }