AthenaMonitoringKernel: Fix string filling with generator
Fix and unit testing for the following use-case:
Monitored::Scalar<std::string>( "DetID", [&](){return "SCT";} );
Filling from a generated string was not supported and instead of a
compiler error it actually made the code crash because it tried to
initialize a string from a double (e.g. std::string m_value(0)
).
P.S.: Using a transformation, e.g.
Monitored::Scalar<std::string>( "DetID", "default", [&](std::string s){return s+"postfix";} );
is still broken. May address that in a separate MR.
cc @tbold