Geo: fix edge case return values for uniformity.
Geo: fix edge case return values for uniformity.

There were two cases outlined in issue #3512 and PR #3551 where
the Geo API returned unexpected results: empty strings where NULL
replies were expected, or a single null reply where an array was
expected. This violates the Redis principle that Redis replies for
existing keys or elements should be indistinguishable.

This is technically an API breakage so will be merged only into 4.0 and
specified in the changelog in the list of breaking compatibilities, even
if it is not very likely that actual code will be affected, hopefully,
since with the past behavior basically there was to acconut for *both*
the possibilities, and the new behavior is always one of the two, but
in a consistent way.

kensou97/redis,oranagra/redis,dreamquster/redis,ofirluzon/redis,kensou97/redis,oranagra/redis,Sciumo/redis,soveran/redis,rogerchina/redis,YongMan/redis,charsyam/redis,Sciumo/redis,rogerchina/redis,arijitvt/redis,yossigo/redis,charsyam/redis,pmem/redis,neomantra/redis,ctripcorp/redis,PKRoma/redis,neomantra/redis,zguangyu/redis,PKRoma/redis,soveran/redis,arijitvt/redis,badboy/redis,dreamquster/redis,ofirluzon/redis,soveran/redis,oranagra/redis,rogerchina/redis,ton31337/redis,yossigo/redis,ctripcorp/redis,nnog/redis,pmem/redis,ton31337/redis,nnog/redis,neomantra/redis,soloestoy/redis,PKRoma/redis,badboy/redis,ton31337/redis,soloestoy/redis,yossigo/redis,badboy/redis,antirez/redis,soveran/redis,charsyam/redis,rogerchina/redis,hgl888/redis,spinlock/redis,GitHubMota/redis,oranagra/redis,PKRoma/redis,kensou97/redis,yossigo/redis,antirez/redis,soloestoy/redis,hgl888/redis,ofirluzon/redis,soloestoy/redis,YongMan/redis,pmem/redis,YongMan/redis,pmem/redis,spinlock/redis,ctripcorp/redis,soloestoy/redis,yossigo/redis,GitHubMota/redis,hgl888/redis,charsyam/redis,Sciumo/redis,zguangyu/redis,antirez/redis,neomantra/redis,dreamquster/redis,nnog/redis,badboy/redis,dreamquster/redis,nnog/redis,arijitvt/redis,arijitvt/redis,neomantra/redis,YongMan/redis,ctripcorp/redis,antirez/redis,charsyam/redis,GitHubMota/redis,ofirluzon/redis,zguangyu/redis,spinlock/redis,oranagra/redis,hgl888/redis,ton31337/redis,PKRoma/redis,GitHubMota/redis,kensou97/redis,Sciumo/redis,zguangyu/redis,ofirluzon/redis,spinlock/redis