Fixed warning for mem checks

(Added check to test if line is gt than 1 character)

PHP Warning:  Undefined array key 1 in /usr/local/ispconfig/server/lib/classes/cron.d/100-monitor_mem_usage.inc.php on line 79

PHP Deprecated:  trim(): Passing null to parameter #1 ($string) of type string is deprecated in /usr/local/ispconfig/server/lib/classes/cron.d/100-monitor_mem_usage.inc.php on line 79

Signed-off-by: Nephiaust <29741794+Nephiaust@users.noreply.github.com>