Don't strip out all empty fragments

Empty fragments may need to be layed out to draw borders, padding/background,
and insertion points.  (Fragments that consist of discardable whitespace and
control characters, on the other hand, can still be discarded.)

This ends up preserving some useless empty fragments.  It's possible we could
avoid this by storing some sort of flag on "important" empty fragments, so we
can discard the rest.

tests/wpt/metadata-css/css21_dev/html4/bidi-011.htm.ini

@@ -1,3 +0,0 @@
-[bidi-011.htm]
-  type: reftest
-  expected: FAIL