Permission denied error running dbic-migration with no target dir specified

Hi John, I was following the tutorial and hit an error running prepare without a target dir specified:

    $ dbic-migration --schema_class Bean::AdcDB::Schema -Ilib prepare
    mkdir /_source: Permission denied at .../lib/perl5/Path/Class/Dir.pm line 154.

Supplying a target_dir (`--target_dir share`) works as intended.

DBIC::Mig::Script passes an empty `script_directory` in the deployment handler opts so DH tries to install to `/` instead of the default `share`.

This happens because all predicate methods on attributes with the MooseX::Attributes::ENV trait return true. This is because MooseX::Attributes::ENV using defaults so the attribute value will be set afterwards (I can't see any way for a default sub to bail and return a 'not set' value)

```perl
# has_* always returns true even though it hasn't been set
DBIx::Class::Migration::Script->new_with_options->has_target_dir();
```

I ran all the tests after the change and they still pass.