Handle explicit case of an empty string

A empty regex string will match no patterns, whereas as find for an empty
string will match any pattern, so to maintain the expected behaviour of
an empty string matching any key, this has to be handled separately

Trigger/TrigSteer/TrigCompositeUtils/TrigCompositeUtils/TrigCompositeUtils.icc

@@ -141,6 +141,9 @@ namespace TrigCompositeUtils {
     }
     auto it = std::remove_if(vector.begin(), vector.end(), [&](const ElementLink<CONTAINER>& el) {
 
+        /// handle explicit case of an empty string
+        if ( containerSGKey=="" ) return false;
+
 	bool newmatch = !std::regex_match( el.dataID(), std::regex(containerSGKey) );
 
 	if ( containerSGKey.find("*")==std::string::npos &&