Fix flushing issue in vtkWeb process at startup

Process launcher for vtkWeb/ParaViewWeb check output stream
to validate the availability of the service. For some reason
the output was not flushed. To force it to be flushed we make
sure we output some generated padding to fill the buffer and
make sure it get flushed so the process launcher could still rely
on reading the process output.

Change-Id: Iedde4e72e27e8fddb3f41be98687b3c1e5ea4207
Merge topic 'vtk-web-flush-start-factory' into master

af9cb388 Fix flushing issue in vtkWeb process at startup