fix(matrix): send fallback media when attachment list is blank (#128479)

fix(matrix): send fallback media when attachment list is blank (#128479)

* fix(matrix): send fallback media when attachment list is blank

* fix(matrix): align media validation and delivery

* fix(matrix): reject whitespace-only reply parts

* fix(matrix): normalize streamed reply media before draft finalization

Co-authored-by: Alix-007 

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Co-authored-by: Peter Steinberger