…or assistant messages with tool calls (#14951)

## background

backport of #14950 from main to release-v6.0

## related issues

backport of #14950
follow-up to #13744

…messages with tool calls (#14950)

## background

#13744 changed `content: text` to `content: text || null` for assistant
messages. this correctly fixed tool-only messages (openai requires
`content: null` when `tool_calls` is present), but also broke the case
where an assistant message has empty text and no tool calls - openai
rejects `content: null` when `tool_calls` is absent with "Invalid value
for 'content': expected a string, got null"

per the [openai api
spec](https://developers.openai.com/api/reference/go/resources/chat/subresources/completions),
content is "required unless `tool_calls` or `function_call` is
specified"

## summary

- only send `null` when `toolCalls.length > 0`, otherwise preserve the
text string
- add regression tests for both `@ai-sdk/openai` and
`@ai-sdk/openai-compatible`

## verification

verified against live openai chat completions api with
`openai.chat('gpt-4o-mini')` - empty assistant content now sends
`content: ""` and returns a valid response

## related issues

follow-up to #13744