Fix regexp check to match '^$'

When ./check_resmon_metric -H myhost -M Core::Fmadm -C failures -m resources -r '^$'
Use of uninitialized value $value in pattern match (m//) at /etc/nagios3/CS.conf.d/check_resmon_metric line 378.
Use of uninitialized value $value in concatenation (.) or string at /etc/nagios3/CS.conf.d/check_resmon_metric line 383.
OK: resources -
To avoid false critical status check if $value is not defined and if so define as empty string