Remove style/class attributes if values are whitespace or empty string

[ekashida]

This is a straggler from #4124

For example:

<div class=""></div>
<div style=" "></div>

In the static-optimized case, these attributes are kept as-is. For the non-static-optimized case, the attributes are removed entirely:

<div></div>
<div></div>

[git2gus]

This issue has been linked to a new work item: W-16772409