I found this in SpecialContributions.php (about line 72):
if ( 0 == $nCur && 0 == $nOld && 0 == $rcrows ) { $wgOut->addHTML( "\n<p>" . wfMsg( "nocontribs" ) . "</p>\n" ); return; }
What I'm missing is the $rcrows variable initialisation. Hopefully php init it with 0, but I'm not sure.
Possibly Lee or Brion have an idea :)