Fix null exception when accessing valueBytes before encodedBytes

[nico-famedly]

Noticed this when doing some weird stuff in a project. If you had not called encodedBytes first, then some object types would throw. You can work around it, but the fix for it is pretty simple :)

[wstrange]

Thanks Nicolas - much appreciated! Can you bump the minor version number in pubspec, and add a changelog entry. Then I'll push a new version to pub

[nico-famedly]

Done! Feel free to squash as you prefer.

(Also thank you for the fast response!)

[wstrange]

New version published. Thanks!!