Doc: Fix empty module header in documentation config

Fixes qdoc warning about

     Could not find the module header in include paths for module "QtLottieAnimation"

Commit 137429647bb5152015a389dd4fbb8898c4644d54 already attempted
to fix this by setting the moduleheader variable to empty, but this
was overriden a few lines below ...

Pick-to: 6.11 6.12
Change-Id: I12f440bb6b583edac65293c8d701d96924da384b
Reviewed-by: Paul Wicking <paul.wicking@qt.io>