parseDouble() fell through to a float cast for ',nan', and
(float) 'nan' evaluates to 0.0 in PHP, so a NaN score silently became
a valid zero. Return NAN instead. The RESP3 specification also notes
that Redis before 7.2 may emit any libc representation of NaN
('-nan', 'NAN', 'nan(char-sequence)') and that clients should handle
them, so those spellings are accepted as well.
The TDigest RESP3 tests asserted 0 or null for empty sketches, which
only passed because of the collapsed 0.0 (null == 0.0 loosely); their
RESP2 counterparts already assert the string 'nan' for the same
replies. They now assert NaN.
Note for reviewers: json_encode() throws on NAN, so consumers who
serialize raw replies must handle it; the previous behavior hid NaN
behind a plausible-looking 0.0 instead.
Sergey Sannikov
3ffaf10e99