Empty <g> nodes should not be removed if they contain a filter, including
filters applied via CSS.

===

<svg viewBox="0 0 50 50" xmlns="http://www.w3.org/2000/svg">
    <filter id="a" x="0" y="0" width="50" height="50" filterUnits="userSpaceOnUse">
        <feFlood flood-color="#aaa"/>
    </filter>
    <mask id="b" x="0" y="0" width="50" height="50">
        <g style="filter: url(#a)"/>
    </mask>
    <text x="16" y="16" style="mask: url(#b)">•ᴗ•</text>
</svg>


@@@

<svg viewBox="0 0 50 50" xmlns="http://www.w3.org/2000/svg">
    <filter id="a" x="0" y="0" width="50" height="50" filterUnits="userSpaceOnUse">
        <feFlood flood-color="#aaa"/>
    </filter>
    <mask id="b" x="0" y="0" width="50" height="50">
        <g style="filter: url(#a)"/>
    </mask>
    <text x="16" y="16" style="mask: url(#b)">•ᴗ•</text>
</svg>