pg:info raises warning with ruby 2.0 when building hash from array with nil values

Ruby 2.0 raises a warning when trying to build a hash from an array with nil values. 
```
> heroku pg:info --app quikly-prod
.heroku/client/lib/heroku/command/pg.rb:338: warning: wrong element type nil at 1 (expected array)
.heroku/client/lib/heroku/command/pg.rb:338: warning: ignoring wrong elements is deprecated, remove them explicitly
.heroku/client/lib/heroku/command/pg.rb:338: warning: this causes ArgumentError in the next release
```

This pull request just compacts the array before creating a hash from it.