Description:
When running the mail merge wizard to update a document that already contains mail merge fields with a new data source, the "Next >" button never is enabled, and it is fully unclear why.

Steps to Reproduce:
1. Open ODT file that already contains mail merge fields and is linked to an ODS file.
2. Try to replace that ODS file by a new one using the mail merge wizard
3. Whatever you select as data source, "Next >" is disabled

Actual Results:
"Next >" is disabled, no info on why

Expected Results:
After selecting a data source, "Next >" should be enabled. If an unsuitable data source is selected, it should be explained what is missing. 


Reproducible: Always


User Profile Reset: No

Additional Info:
Version: 7.6.4.1 (X86_64) / LibreOffice Community
Build ID: 60(Build:1)
CPU threads: 8; OS: Linux 6.5; UI render: default; VCL: gtk3
Locale: de-DE (de_DE.UTF-8); UI: de-DE
Ubuntu package version: 4:7.6.4-0ubuntu0.22.04.1~lo1
Calc: threaded