Prevent NRE when Android SDK is not found (#73)

Fixes: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/938504

In dc00a39, we added a search path for the NDK which is now
*inside* the Android SDK. However, this change could produce a
`NullReferenceException` if the Android SDK is not found.

In cases where an Android SDK is not found, and the `AndroidSdkPath`
property is null, we still go on to use the null property when
locating the Android NDK:

    var sdks = GetAllAvailableAndroidSdks().ToList();
    sdks.Add(AndroidSdkPath);
    foreach(var sdk in sdks.Distinct())
        if (Directory.Exists(ndk = Path.Combine(sdk, "ndk-bundle")))

The `Path.Combine` will throw if you pass in a null.

This fix doesn't *solve* the exception, as you will still get an
`InvalidOperationException` for the missing Android SDK. However, that
is certainly a more descriptive error message than a
`NullReferenceException`.

src/Xamarin.Android.Tools.AndroidSdk/Sdks/AndroidSdkWindows.cs

@@ -241,7 +241,8 @@ protected override IEnumerable<string> GetAllAvailableAndroidNdks ()
 			string ndk;
 
 			var sdks = GetAllAvailableAndroidSdks().ToList();
-			sdks.Add(AndroidSdkPath);
+			if (!string.IsNullOrEmpty(AndroidSdkPath))
+				sdks.Add(AndroidSdkPath);
 
 			foreach(var sdk in sdks.Distinct())
 				if (Directory.Exists(ndk = Path.Combine(sdk, "ndk-bundle")))