Buttons should only be wrapped in a column when they are not placed in a column already

[janus-reith]

A button will always be wrapped in a new column:

newspack-newsletters/includes/class-newspack-newsletters-renderer.php

Line 363 in e7b1f83

$block_mjml_markup .= '<mj-column ' . self::array_to_attributes( $column_attrs ) . '><mj-button ' . self::array_to_attributes( $button_attrs ) . ">$text</mj-button></mj-column>";

This leads to breaking layouts when a button is not placed as top-level element but inside a column.
Check is_in_column and only wrap the button if false.