[monitoring-plugin-perl] output empty values as "U"

[daku3649]

 Module: monitoring-plugin-perl
 Branch: master
 Commit: 4434110d88683681655b4609782d317504a3f662
 Author: daku3649 <daku8938 at gmx.de>
   Date: Thu Dec  3 12:35:19 2015 +0100
    URL: https://www.monitoring-plugins.org/repositories/monitoring-plugin-perl/commit/?id=4434110

output empty values as "U"

output empty performance data value as value "U" to "indicate that the actual value couldn't be determined" (defined in https://nagios-plugins.org/doc/guidelines.html#AEN200) and do valid output

---

 lib/Monitoring/Plugin/Performance.pm | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/lib/Monitoring/Plugin/Performance.pm b/lib/Monitoring/Plugin/Performance.pm
index f1e7a16..0871042 100644
--- a/lib/Monitoring/Plugin/Performance.pm
+++ b/lib/Monitoring/Plugin/Performance.pm
@@ -64,9 +64,16 @@ sub perfoutput {
 	if ($label =~ / /) {
 		$label = "'$label'";
 	}
+	
+	my $value = $self->value;
+	# To prevent invalid output, we change empty value to value "U"
+	if ($value eq '') {
+		$value = 'U';
+    }
+    
     my $out = sprintf "%s=%s%s;%s;%s;%s;%s",
         $label,
-        $self->value,
+        $value,
         $self->_nvl($self->uom),
         $self->_nvl($self->warning),
         $self->_nvl($self->critical),