unicode: fix SpecialCase to follow its docs & respect explict no-op mappings

unicode: fix SpecialCase to follow its docs & respect explict no-op mappings

If SpecialCase contains an explicit CaseRange with zero deltas,
respect those and don't fall back to the default behavior.

Fixes #25636

Change-Id: Ic554c6b3dd462b1b39c75194eec469b6ff4aa55b
Reviewed-on: https://go-review.googlesource.com/117155
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>