- prior test encoders were never invoked for `null` or empty `string`s e.g.
``` c#
public static void HtmlEncode(this IHtmlEncoder htmlEncoder, string value, TextWriter output)
{
...
if (!String.IsNullOrEmpty(value))
{
htmlEncoder.HtmlEncode(value, 0, value.Length, output);
}
}
```
- add missing `null` checks and handle `string.Empty` in `TextWriter output, string value, ...` overloads
- better match for the underlying `TextEncoder` behaviour
- `EncoderExtensions` provided an API like `TextEncoder.Encode(TextWriter output, string value)`
- that method calls `Encode(TextWriter output, string value, int startIndex, int characterCount)`
Doug Bunting