CSL - Fix bug when inserting reference coalescing before first entry

[subhramit]

Follow-up to: #11712
Fixes the following edge-case bugs:

1. Inserting a new reference just before the reference numbered "1" without any space or line break or text in between (coalescing case 1)

bug1.mp4

2. Inserting a new reference before the reference numbered "1" with a paragraph break [shift+enter] (not a line break [enter]) (coalescing case 2)

bug2.mp4

Both cases used to throw an exception and did not update the old "1" after the new reference to "2".

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[subhramit]

Just came to my notice that the bibliography does not update with the new entry (coalesced before the old entry).

This is unrelated, and is because of the bleed-out of the old reference mark (the new one becomes a part of the old 1's reference mark as well). This is due to no proper manual space/break, and the fact that if we open a fresh document and insert a reference, smart spaces are not inserted as well (as there is literally no text in the document - hence practically unlikely to occur for a CSL user. It is also unlikely that they will try placing a citation before the first citation without any text or break, and this happens only in the case of the first citation).

Hence, we can let this PR in, as it at least avoids the exception.