tls: Try empty name if no matches for getting config during handshake

See discussion on #2015; the initial change had removed this check, and
I can't remember why I removed it or if it was accidental. Anyway, it's
back now.

caddytls/handshake.go

@@ -59,6 +59,14 @@ func (cg configGroup) getConfig(name string) *Config {
 		}
 	}
 
+	// try a config that serves all names (this
+	// is basically the same as a config defined
+	// for "*" -- I think -- but the above loop
+	// doesn't try an empty string)
+	if config, ok := cg[""]; ok {
+		return config
+	}
+
 	// no matches, so just serve up a random config
 	for _, config := range cg {
 		return config