Fix wrong element warnings coming from pg command

Calling `Hash::[]` with an array containing nil will output warnings on Ruby 2.x:

    .../command/pg.rb:338: warning: wrong element type nil at 1 (expected array)
    .../command/pg.rb:338: warning: ignoring wrong elements is deprecated, remove them explicitly
    .../command/pg.rb:338: warning: this causes ArgumentError in the next release

Fix by calling #compact on the array to remove the nils.