Fix Alias.all

Hi,
I open this PR to fix a check in alias.rb at line 97. This fix avoid raising of "NoMethodError: undefined method `empty?' for nil:NilClass" when the key 'aliases' is not present in value hash.

I add the fix because I am receiving that error when I call: Tire.index(index).aliases

NoMethodError: undefined method `empty?' for nil:NilClass

[GEM_ROOT]/gems/tire-0.5.5/lib/tire/alias.rb:97:in `block in all'
[GEM_ROOT]/gems/tire-0.5.5/lib/tire/alias.rb:95:in `each'
[GEM_ROOT]/gems/tire-0.5.5/lib/tire/alias.rb:95:in `inject'
[GEM_ROOT]/gems/tire-0.5.5/lib/tire/alias.rb:95:in `all'
[GEM_ROOT]/gems/tire-0.5.5/lib/tire/index.rb:52:in `aliases'
...


Unfortunately I was unable to add a test to cover this behaviour: I tried to change index_alias_test and index_test without success.