Return null on Windows if address could not be found

This is in line with the Linux implementation and the docs, see also #7.
This is a breaking change as what has thrown an exception before now returns null.

src/ArpLookup/WindowsLookupService.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Net;
@@ -26,7 +26,7 @@ internal static class WindowsLookupService
         /// <param name="ip">The <see cref="IPAddress"/> to pass to the Win32 API.</param>
         /// <exception cref="Win32Exception">If IpHlpApi.SendARP returns non-zero.</exception>
         /// <returns>A <see cref="PhysicalAddress"/> instance that represents the address found by IpHlpApi.SendARP.</returns>
-        public static PhysicalAddress Lookup(IPAddress ip)
+        public static PhysicalAddress? Lookup(IPAddress ip)
         {
             _ = ip ?? throw new ArgumentNullException(nameof(ip));
             if (!IsSupported)
@@ -46,12 +46,18 @@ public static PhysicalAddress Lookup(IPAddress ip)
             {
                 return new PhysicalAddress(addr);
             }
+            else if (res == NativeMethods.ERROR_BAD_NET_NAME)
+            {
+                return null;
+            }
 
             throw new Win32Exception(res);
         }
 
         private static class NativeMethods
         {
+            public const int ERROR_BAD_NET_NAME = 67;
+
             // based on code generated by https://github.com/microsoft/CsWin32, modified
 
             /// <summary>The SendARP function sends an Address Resolution Protocol (ARP) request to obtain the physical address that corresponds to the specified destination IPv4 address.</summary>

version.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://raw.githubusercontent.com/dotnet/Nerdbank.GitVersioning/master/src/NerdBank.GitVersioning/version.schema.json",
-  "version": "1.3-alpha",
+  "version": "2.0-alpha",
   "publicReleaseRefSpec": [
     "^refs/heads/release/v\\d+(?:\\.\\d+)?$"
   ],
@@ -12,4 +12,4 @@
   "release": {
     "branchName": "release/v{version}"
   }
-}
+}