fix: distinguish empty-string content from None in Document auto-id

[ompatil1906]

Problem

Document._create_id normalized empty content with text = self.content or None. Consequently Document(content="") and Document(content=None) produced identical auto-generated IDs whenever the rest of their fields matched.

This caused:

• a misleading DuplicateDocumentError when both were written in the same batch, and
• silent ID collisions (two documents stored under one ID) when written separately.

Empty string is a valid content value (str | None), so the documents are semantically different.

Fixes #13234

Solution

Use text = self.content directly. The stringification then distinguishes "" from None while leaving the stable ID of a completely empty Document() unchanged.

Verification

• Added regression test test_empty_string_content_has_distinct_id_from_none that asserts the two IDs differ and that the content=None ID remains the previously published value.
• Ran the new test together with the existing test_init (both passed).
• Manually confirmed the IDs now differ.

Release note added under releasenotes/notes/.

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[github-actions]

Hi @ompatil1906, thanks for your interest in contributing to Haystack! 🙏

⚠️ Issue #13234 is already being addressed by open pull request(s) #13235. Before opening a PR for an issue, please check whether a PR is already linked to it, and consider contributing to the existing PR instead. We may close duplicate PRs to keep the review queue manageable.

This is an automated message to help us keep the review queue healthy.

[HaystackBot]

Hi @ompatil1906, thanks a lot for your contribution! 🙏

We noticed that the Contributor License Agreement (CLA) check (license/cla) hasn't passed yet, so we've temporarily moved this PR to draft and paused the review assignment.

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[HaystackBot]

Thanks for signing the CLA, @ompatil1906! 🎉 This PR is now ready for review again and the reviewer has been re-assigned.

[sjrl]

Thanks for the PR! We've decided not to change this behavior since it would change the auto-generated IDs of existing documents with empty content. See #13234 for details. Closing.