If GT/LT fails the operation we need to reply with
nill (like failure due to NX).
Other changes:
Add the missing $encoding suffix to many zset tests
Note: there's a behavior change just in case of INCR + GT/LT that fails.
The old code was replying with the wrong (rejected) score, and now it'll reply with nil.
Note that that's anyway a corner case so this "behavior change" shouldn't have too much affect.
Using GT/LT with INCR has a predictable result even before we run the command
(INCR GT will only only / always fail if the increment is negative).
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