In previous releases, if -asmap was specified without a filename, this would try to load an ip_asn.map data file. Now loading an asmap file requires an explicit filename like -asmap=ip_asn.map. This change was made to make the option easier to understand, because it was confusing for there to be a default filename not actually loaded by default (bitcoin#33386). Also this change makes the option more future-proof, because in upcoming releases, specifying -asmap will load embedded asmap data instead of an external file (bitcoin#28792).