Fix nil being passed to Regex.match? in Format validator

Previously this would error out with nil being passed to Regex.match?

```elixir
defmodule User do
  use Vex.Struct

  defstruct email: nil

  validates :email, presence: true, format: ~r/^\S+@\S+\.\S+$/
end
```

This PR converts to a string to fix the issue. I also added a testcase for this scenario. Thanks!