modulecmd: use of unset variable fixed

Pmodules/modulecmd.bash.in

@@ -1993,13 +1993,15 @@ subcommand_unuse() {
 					 "${arg}"
 			fi
 
-                        local var="PMODULES_LOADED_${arg^^}"
-                        if [[ -n "${!var}" ]]; then
-                                std::die 3 "%s %s: %s -- %s" \
-                                         "${CMD}" "${SubCommand}" \
-                                         "cannot remove group due to loaded modules" \
-                                         "${arg}"
-                        fi
+			if [[ -v PMODULES_LOADED_${arg^^} ]]; then 
+				local var="PMODULES_LOADED_${arg^^}"
+				if [[ -n "${!var}" ]]; then
+					std::die 3 "%s %s: %s -- %s" \
+						 "${CMD}" "${SubCommand}" \
+						 "cannot remove group due to loaded modules" \
+						 "${arg}"
+				fi
+			fi
                         std::remove_path UsedGroups "${arg}"
                         local overlay
                         for overlay in "${UsedOverlays[@]}"; do