Fix some validators for nil values

Using the real entities for the tests also uncovered some bugs where for
example empty strings are converted to nil and the validation wasn't
invalid in this case, but should be.

lib/diaspora_federation/entities/account_migration.rb

@@ -43,7 +43,7 @@ module DiasporaFederation
       # Returns diaspora* ID of the new person identity.
       # @return [String] diaspora* ID of the new person identity
       def new_identity
-        profile.author
+        profile.author if profile
       end
 
       # @return [String] string representation of this object

lib/diaspora_federation/validators/conversation_validator.rb

@@ -9,7 +9,7 @@ module DiasporaFederation
 
       rule :subject, [:not_empty, length: {maximum: 255}]
 
-      rule :participants, diaspora_id_list: {minimum: 2}
+      rule :participants, [:not_empty, diaspora_id_list: {minimum: 2}]
       rule :messages, :not_nil
     end
   end

lib/diaspora_federation/validators/event_participation_validator.rb

@@ -6,7 +6,7 @@ module DiasporaFederation
 
       include RelayableValidator
 
-      rule :status, regular_expression: {regex: /\A(accepted|declined|tentative)\z/}
+      rule :status, [:not_empty, regular_expression: {regex: /\A(accepted|declined|tentative)\z/}]
     end
   end
 end

spec/lib/diaspora_federation/validators/web_finger_validator_spec.rb

@@ -27,7 +27,7 @@ module DiasporaFederation
       describe "##{prop}" do
         it_behaves_like "a property with a value validation/restriction" do
           let(:property) { prop }
-          let(:wrong_values) { ["", "https://asdf$%.com", "example.com"] }
+          let(:wrong_values) { %w[https://asdf$.com example.com] }
           let(:correct_values) { [nil] }
         end