Fix NULL check issues (HIGH priority)

Added validation checks for potentially empty variables before use
to prevent errors and unsafe operations.

WordPress Cron Manager (5 fixes):
- Added site_path validation after dirname operations
- Prevents using empty paths in cd commands and file operations
- Pattern: Check [ -z "$site_path" ] before use

Bot Analyzer:
- Quoted TEMP_DIR in trap command for safety

Hardware Health Check:
- Quoted MESSAGES_CACHE in trap command for safety

Note: 5 issues flagged in toolkit-qa-check.sh were false positives
(echo statements demonstrating bad patterns, not actual code issues)

modules/performance/hardware-health-check.sh

@@ -1700,7 +1700,7 @@ main() {
         touch "$MESSAGES_CACHE"
     fi
     # Cleanup cache on exit
-    trap "rm -f $MESSAGES_CACHE" EXIT
+    trap "rm -f \"$MESSAGES_CACHE\"" EXIT
 
     # Run diagnostics with progress indicators
     echo -e "${YELLOW}[1/11]${NC} Analyzing disk SMART status and predictive failure indicators..."