Bug#787967: pysph: FTBFS when not building -doc package

[Aaron M. Ucko]

Source: pysph
Version: 0~20150606.gitfa26de9-1
Severity: serious
Justification: fails to build from source

Thanks for taking care of the build errors I previously reported; I'm
pleased to confirm that neither is still a problem.  Nevertheless,
automatic builds of pysph are still failing because they don't cover
the architecture-independent -doc package:

  find /«PKGBUILDDIR»/debian/pysph-doc/usr/share/doc/pysph-doc/html -type f -print0 | xargs -0 sed -i 's/http:\/\/cdn.mathjax.org\/mathjax\/latest/\/usr\/share\/javascript\/mathjax/g'
  find: `/«PKGBUILDDIR»/debian/pysph-doc/usr/share/doc/pysph-doc/html': No such file or directory
  sed: no input files
  make[1]: *** [override_dh_installdocs] Error 123

Could you please take a look?  One possible fix would be to take
advantage of xargs's --no-run-if-empty flag.  You can reproduce the
problem by running dpkg-buildpackage (or your favorite wrapper) with
the -B flag.

Thanks!