Fix label for not matching input id when collection value is nil

[55728]

Fixes #1840.

Problem

When a collection contains a nil value, the generated label's for attribute does not match the corresponding input's id:

<%= form.input :my_method, as: :radio_buttons,
      collection: [['Yes', true], ['Undefined', nil]] %>

produces:

<!-- ✅ "true" — matches -->
<input id="model_my_method_true" type="radio" value="true" />
<label for="model_my_method_true">Yes</label>

<!-- ❌ nil — trailing underscore mismatch -->
<input id="model_my_method" type="radio" value="" />
<label for="model_my_method_">Undefined</label>

Clicking the "Undefined" label does not select its radio button because the for / id values differ by a single trailing underscore.

Cause

Rails' sanitize_attribute_name unconditionally interpolates an underscore separator:

def sanitize_attribute_name(value)
  "#{sanitized_method_name}_#{sanitized_value(value)}"
end

When value is nil, sanitized_value(nil) returns "", producing "my_method_" (trailing underscore). This value is passed to the label builder as the method name, which generates for="model_my_method_".

Meanwhile, the radio button's id is generated by add_default_name_and_id_for_value, which has a separate nil-check that skips the value suffix entirely, producing id="model_my_method" (no trailing underscore).

Fix

Override sanitize_attribute_name in SimpleForm::Tags::CollectionExtensions to skip the underscore when the sanitized value is empty:

def sanitize_attribute_name(value)
  sanitized = sanitized_value(value)

  if sanitized.empty?
    sanitized_method_name.dup
  else
    "#{sanitized_method_name}_#{sanitized}"
  end
end

This aligns the label's for with the input's id for both collection_radio_buttons and collection_check_boxes, in both inline and nested boolean styles.