Bug fix: Filter out empty names from context list

[catsby]

On some operating systems (namely macOS), it's possible for something akin to a .DS_Store file to exist in the directory where contexts are stored. If so, then when a user runs waypoint context list an internal method nameFromPath returns an empty string for said file as it doesn't have a normal file extension. context list then tries to load a non-existing /.hcl file and errors like so:

$ waypoint context list 
! Error loading context "": <nil>: Configuration file not found; The configuration file /Users/clint/Library/Preferences/waypoint/context/.hcl does not exist.

In this PR we simply filter out any non .hcl files before we call nameFromPath, which should save us from any other possible .dotfiles or other files that don't have a file extension at all. It would be odd to have any others in that folder but this should hopefully Catch'em All

[briancain]

Looks good to me!