None check for encoder

[sfilios]

In the case that BartForConditionalGeneration decoder is being used without an encoder, this change maintains the ability to resize embeddings.

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ x] Did you read the contributor guideline, Pull Request section?
• [ ] Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• [Not Necessary] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [Not Necessary] Did you write any new necessary tests?

@ArthurZucker and @younesbelkada

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[ArthurZucker]

Hey! As you can see from the red tests, this cannot be merged as it is breaking a lot of the API 😅