Don't update row baseline if cell is empty (#31831)

Gecko, Blink and WebKit agree that the if a row only has empty cells,
its baseline should be at the bottom, not at the top.

There isn't interoperability when the cells are just empty-ish, so this
patch takes the simplest approach, aligning with Blink: any out-of-flow
or in-flow content other than collapsed whitespace counts as not empty.

tests/wpt/meta/MANIFEST.json

@@ -106867,6 +106867,19 @@
         {}
        ]
       ],
+      "table-vertical-align-baseline-008.xht": [
+       "60079c7ce26491a0d91c8e818a1a7ead406c3a20",
+       [
+        null,
+        [
+         [
+          "/css/CSS2/reference/ref-filled-green-100px-square.xht",
+          "=="
+         ]
+        ],
+        {}
+       ]
+      ],
       "table-visual-layout-017.xht": [
        "25067cb68385a520a10a31949d742b520c7e9cd6",
        [