WYSIWYG with type multi-line adds break-line when cleared

[MariaFdezJ]

Description

When WYSIWYG input, with type: multi-line changes from text to an empty string, it saves with an extra <br />

Additional Context

Currently, we are trimming the break lines, but when the text is erased, the value is actually <p><br /></p>.

• We can either verify if the text is exactly that in trimLinebreaks: https://github.com/clay/clay-kiln/blob/master/inputs/wysiwyg-sanitize.js#L67
• Or do it inside sanitizeBlockHTML: https://github.com/clay/clay-kiln/blob/master/inputs/wysiwyg-sanitize.js#L160