[15.0][Fix] mail: _migrate_placeholder_html should remove '| safe'

[cuongnmtm]

This will fix the issue when Odoo renders this wrong syntax

Content in email template body v14:

${object.company_id.email | safe}

Content in email template body v15 (after migrate data):

<t t-out="object.company_id.email | safe">

Error:

odoo.addons.base.models.ir_qweb.QWebException: Error while render the template
TypeError: unsupported operand type(s) for |: 'str' and 'NoneType'