Fix FrozenError from nil.to_s << on Ruby 2.7

[r7kamura]

When I tried to upgrade Ruby version from 2.6 to 2.7, I had a problem with FrozenError from #nested_form_for.

capture may return nil, and nil.to_s is frozen from Ruby 2.7.

Ruby 2.6

$ ruby -e 'p nil.to_s.frozen?'
false

Ruby 2.7

$ ruby -e 'p nil.to_s.frozen?'
true