Allow non-string types for applicant attributes

[zachmargolis]

Context:

• I introduced a boolean applicant attribute as a feature flag

Problem:

• The "empty?" check we do blows up on non-string types
• Spec failure looks like:

• 1) Proofer::Base#validate_attributes when optional attributes are booleans does not raise
     Failure/Error: expect { subject }.not_to raise_exception
     
       expected no Exception, got #<NoMethodError: undefined method `empty?' for true:TrueClass> with backtrace:
         # ./lib/proofer/base.rb:96:in `blank?'
         # ./lib/proofer/base.rb:65:in `block in validate_attributes'
         # ./lib/proofer/base.rb:65:in `select'
         # ./lib/proofer/base.rb:65:in `validate_attributes'
         # ./spec/lib/proofer/base_spec.rb:109:in `block (3 levels) in <top (required)>'
         # ./spec/lib/proofer/base_spec.rb:140:in `block (5 levels) in <top (required)>'
         # ./spec/lib/proofer/base_spec.rb:140:in `block (4 levels) in <top (required)>'
     # ./spec/lib/proofer/base_spec.rb:140:in `block (4 levels) in <top (required)>'
  

Solution:

• Make the empty check more flexible WRT to object types

[stevegsa]

LGTM

[orenyk]