fix: prevent error if plasmax11.desktop is missing

Signed-off-by: Boris Yumankulov <boria138@altlinux.org>

usr/bin/portproton-session-select

@@ -66,8 +66,11 @@ if [[ -z $SENTINEL_CREATED ]]; then
   export SENTINEL_CREATED=1
 fi
 
-if [[ $(grep -E '^X-KDE-PluginInfo-Version=' /usr/share/xsessions/plasmax11.desktop | cut -d'=' -f2) =~ 6.* ]] ; then
-  KDE6="true"
+if [[ -f /usr/share/xsessions/plasmax11.desktop ]]; then
+  version=$(grep -E '^X-KDE-PluginInfo-Version=' /usr/share/xsessions/plasmax11.desktop | cut -d'=' -f2)
+  if [[ $version =~ ^6.* ]]; then
+    KDE6="true"
+  fi
 fi
 
 case "$session" in