Detect empty databases and report sensible warning.
Detect empty databases and report sensible warning.

On a fresh installation with no reports yet, the cron job emit lots of gnuplot errors,
causing new site admins to worry what is going wrong.  Detect the situation and
give a more clear warning about the problem instead.

nzherald/alaveteli,andreicristianpetcu/alaveteli_old,Br3nda/alaveteli,petterreinholdtsen/alaveteli,andreicristianpetcu/alaveteli_old,andreicristianpetcu/alaveteli,petterreinholdtsen/alaveteli,andreicristianpetcu/alaveteli_old,Br3nda/alaveteli,4bic/alaveteli,andreicristianpetcu/alaveteli_old,4bic/alaveteli,andreicristianpetcu/alaveteli,nzherald/alaveteli,nzherald/alaveteli,petterreinholdtsen/alaveteli,petterreinholdtsen/alaveteli,Br3nda/alaveteli,4bic/alaveteli,nzherald/alaveteli,andreicristianpetcu/alaveteli,andreicristianpetcu/alaveteli_old,Br3nda/alaveteli,4bic/alaveteli,Br3nda/alaveteli,4bic/alaveteli,nzherald/alaveteli,andreicristianpetcu/alaveteli,petterreinholdtsen/alaveteli,andreicristianpetcu/alaveteli