Check if defaults file exists during installation

If a desktop file is installed with xdg-desktop-menu install, the script tries to unconditionally access $XDG_DATA_HOME/applications/defaults.list, which may be missing. This is the case when installing into a staging directory during the FreeBSD package building process. In that case, the call to grep blocks indefinitely, because grep tries to read from stdin instead. Check for existence of the defaults file before calling grep on the defaults file.

Closes #119 (closed)