Only try to set plugin paths if cadence is installed

This fixes issues with cadence in debian distros,
after being removed the /etc files still remain leaving *_PATH empty

data/70cadence-plugin-paths

@@ -3,9 +3,13 @@
 
 INSTALL_PREFIX="X-PREFIX-X"
 
+if [ -f $INSTALL_PREFIX/bin/cadence-session-start ]; then
+
 export LADSPA_PATH=`$INSTALL_PREFIX/bin/cadence-session-start --printLADSPA_PATH`
 export DSSI_PATH=`$INSTALL_PREFIX/bin/cadence-session-start --printDSSI_PATH`
 export LV2_PATH=`$INSTALL_PREFIX/bin/cadence-session-start --printLV2_PATH`
 export VST_PATH=`$INSTALL_PREFIX/bin/cadence-session-start --printVST_PATH`
 
+fi
+
 unset INSTALL_PREFIX