Bummer. I just checked the docs; it is a required field in the schema. Apologies for that.
Lacking any other registry value that provides a pathname without the terminal filename, the other option to consider is to use just the File Version rule.
Set "Common Path" = PROGRAM_FILES
Set "Path" = ..\Program Files (x86)\Trillian\Trillian.exe
and Comparison and Version as you otherwise would.