Handle nil values in Grape length limit validator

Changelog: fixed

lib/api/validations/validators/limit.rb

@@ -7,7 +7,7 @@ module API
         def validate_param!(attr_name, params)
           value = params[attr_name]
 
-          return if value.size <= @option
+          return if value.nil? || value.size <= @option
 
           raise Grape::Exceptions::Validation.new(
             params: [@scope.full_name(attr_name)],

spec/lib/api/validations/validators/limit_spec.rb

@@ -22,4 +22,10 @@ RSpec.describe API::Validations::Validators::Limit do
       expect_validation_error('test' => "#{'a' * 256}")
     end
   end
+
+  context 'value is nil' do
+    it 'does not raise a validation error' do
+      expect_no_validation_error('test' => nil)
+    end
+  end
 end