fix(generative-ai-dart) & fix(firebase_vertexai): null role on ChatSession history

[Flucadetena]

Fixes and closes issue #197

I extended the existing condition to take into account blocked Content with null role and avoid adding it to the history.

There is a preexisting "TODO" asking if the role should be appended before adding the content to the history. I did not changed or remove the comment, although in my tests a could not reproduce a case where the role was not user or model unless the responses had been blocked for Safety or other reasons.

When blocked the text part is empty and would not make sense to add it to the history. So no sure the appending the role is needed.

With the check the model keeps working fine and the developer can handle the Exception as it pleases, either retrying the generation or adding a default answer by the model so the user can rephrase and retry.

Happy to give more information or make any changes. Hope it helps, thanks for the great work.

[natebosch]

Thanks for the PR!

I landed an alternate version that defaults the role instead of omitting the content.
#205