Return nil when getting a version

If there's no log_data on a model instance and caller tries to get a
version of the model using `#at_version(v)` or `#at(version: v)`, return
`nil` instead of raising `NoMethodError` (undefined method `version' for
nil:NilClass).

lib/logidze/model.rb

@@ -114,6 +114,7 @@ def at!(time: nil, version: nil)
 
     # Return a dirty copy of specified version of record
     def at_version(version)
+      return nil unless log_data
       return self if log_data.version == version
 
       log_entry = log_data.find_by_version(version)

spec/logidze/model_spec.rb

@@ -96,6 +96,11 @@
       expect(user_old.age).to eq 0
       expect(user_old.active).to eq true
     end
+
+    it "returns nil if log_data is nil" do
+      user.log_data = nil
+      expect(user.at(version: 1)).to be_nil
+    end
   end
 
   describe "#at!" do