<td></td><td>foo</td>
I would like to return ['', 'foo'] but libxml's xpath //td/text() returns just ['foo']. How do I find the empty tag as '' instead of (not matched)?
As long as you are selecting text nodes specifically, you can't. Because there simply is no text node in the first <td>.
When you change your XPath expression to '//td', you get the two <td> nodes. Use their text value in further processing.