Fix NULL check issues (5 HIGH issues resolved)

Added proper null/empty checks and variable quoting in 3 files:

1. wordpress-cron-manager.sh (2 issues):
   - Added validation for $site_path before use
   - Quoted variable in cron command to prevent word splitting
   - Lines 446-449: Check if path is empty or invalid before processing

2. malware-scanner.sh (1 issue):
   - Added safety check for $SCAN_DIR before suggesting rm -rf command
   - Prevents dangerous rm operations if variable is empty or root
   - Line 1583-1585: Guard against accidental deletions

3. mysql-restore-to-sql.sh (2 issues):
   - Quoted $datadir in echo statements showing manual commands
   - Lines 426, 441, 444, 447: Proper quoting in examples

Impact: Prevents potential issues from empty/undefined variables

modules/security/malware-scanner.sh

@@ -1580,7 +1580,9 @@ fi
 echo "You can:"
 echo "  • Review logs: ls $LOG_DIR"
 echo "  • View summary: cat $SUMMARY_FILE"
-echo "  • Delete scan directory manually: rm -rf $SCAN_DIR"
+if [ -n "$SCAN_DIR" ] && [[ "$SCAN_DIR" != "/" ]]; then
+    echo "  • Delete scan directory manually: rm -rf \"$SCAN_DIR\""
+fi
 echo ""
 echo "Press Ctrl+A then D to detach from this screen session,"
 echo "or press Enter to open an interactive shell in this session..."