Fix : empty environment bug

[hernoufM]

This PR fixes tool's behavior with regard to environment to match what it describes in manual :

--environment[=VAL] (default=)
     ...
     If set without argument, an empty environment used: this can be 
     used to ensure the generated bundle won't have arch or OS constraints.

Before that, the empty environment was perceived literally as a filter that filtered out all packages that had at least some availability constraints.

[rjbou]

Tiny changes, otherwise lgtm!