You're right, if blank last names are valid for login on OpenSim grids, this would be incompatible as blank would be automatically replaced by "Resident". Though I couldn't find the documentation about OpenSim login.
Are blank last names really allowed for login on OpenSim grids.
Even if they are valid, trying to use a blank last name in Cool VL Viewer 1.25.0.30 would result in this:
It seems that Cool VL Viewer blocks the attemt to use a blank last name anyway.
Instead of simply sending the user back to login prompt, the viewer could make a "smart" guess about what the user might have had in mind with this usecase. -> login to Second Life with a username.
This could be easily archived by adding the second branch of this if-statement.
llpanellogin.cpp from line 1095:
Note: This is just a change of the user experience. The undelying login procedures are left untouched.