EZP-23701: table cell valign property in XmlText is not properly rendered

JIRA: https://jira.ez.no/browse/EZP-23701

# Description

The table cell vertical align property is not properly taken into account by the XmlText XSLT file. This was happening because, the `valign` is in the `custom` namespace and the XSLT was looking for the attribute in the default namespace.
Even if this part is unit tested, the bug was hidden because the output fixtures were wrong.

# Tests

unit tests + manual tests