TF: Merge PT and TF behavior for Bart when no decoder_input_ids are passed
# What does this PR do?

The main model for TF and PT have a different behavior when no `decoder_input_ids` are passed. Because of that, the same model with the same inputs has different outputs in the two platforms, in this specific input case.

Looking at the git blame, it seems like [this](https://github.com/huggingface/transformers/blob/main/src/transformers/models/bart/modeling_bart.py#L1202) `if` branch in PT has added after the TF model was added, and we possibly forgot to port it back.

Notes:
1. All slow tests for Bart were run locally;
2. This was caught with the [updated](https://github.com/huggingface/transformers/pull/17588) `pt-to-tf` CLI, which added much stricter tests.
_The documentation is not available anymore as the PR was closed or merged._