Models without `error` hash generate `undefined method '[]' for nil:NilClass`

Valid form elements generate `undefined method '[]' for nil:NilClass` when the form's model does not include an `errors` hash.

There are some scenarios when rails forms models are not set from an ActiveRecord model. In these scenarios, unless they emulate the `errors` hash produced by ActiveRecord, the rendering of the form element will result in the aforementioned error.

**Example:**
```
<%= bootstrap_form_with(model: my_model) do |f| %>
<%= f.text_field :top_level_field %>
<%= f.fields_for :nested_hash, OpenStruct.new(my_model.nested_hash) do |ff| %>
<%= ff.text_field :nested_hash_field %>
<% end %>
<%= f.primary "Save" %>
<% end %>
```

In this scenario, with vanilla rails form elements, OpenStruct provides all the necessary methods for the `nested_hash_field` property to be rendered correctly. Using `comfy-bootstrap-form` this will generate an error unless the `nested_hash` includes an `errors` property.

**Suggested Fix**

The `draw_errors` method in `form_builder.rb` already includes a fast-return path if the model is `nil`. The criteria should this should be expanded to also return if the `errors` hash is not present.