Auto merge of #16637 - bzbarsky:probing-lazy-pseudos, r=emilio

Fix Stylist::lazily_compute_pseudo_element_style to return None when it should.

We have a fast path to return None if we have no rules for the pseudo-element at
all, but we should also return None if we have no _matching_ rules for it for
the given originating element.  This is relied on by consumers like
Servo_ResolvePseudoStyle, which needs to be able to detect the "no styles for
this pseudo-element for this originating element case", to support probing for
whether work for a specific pseudo-element should be done at all.

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