[6.x] Fix blank element source headings

[riasvdv]

• Preserve blank element source heading values when source settings are saved.
• Normalize existing heading source configs that are missing the heading key so the element sources backend returns valid data.
• Add regression coverage for saving and returning blank headings.

Root cause

In 6.x, Laravel request handling converts empty string form values to null. Blank source headings were therefore saved without a heading key, and the unchanged _elements/sources.twig template later failed when rendering that malformed backend data.

Related issues

Fixes #18889

[linear-code]

CMS-2095