…tant messages (#13744)

## background

when an assistant message contains only tool-call parts (no text),
`convertToOpenAICompatibleChatMessages` and
`convertToOpenAIChatMessages` send `content: ""`. providers backed by
AWS Bedrock reject this with `ValidationException: messages: text
content blocks must be non-empty`

## summary

- change `content: text` to `content: text || null` for assistant
messages in both `@ai-sdk/openai-compatible` and `@ai-sdk/openai`
- update tests to expect `null` instead of `""` for tool-only assistant
messages

## checklist

- [x] tests have been added / updated (for bug fixes / features)
- [ ] documentation has been added / updated (for bug fixes / features)
- [x] a _patch_ changeset for relevant packages has been added (run
`pnpm changeset` in root)
- [x] i have reviewed this pull request (self-review)

## Future work

The same `content: text` pattern exists in `@ai-sdk/xai`,
`@ai-sdk/deepseek`, `@ai-sdk/groq`, and `@ai-sdk/mistral` and will hit
the same Bedrock error under equivalent routing. Tracked separately in
#14612.

## related issues

fixes #13466