adlist: fix listJoin() in the case the second list is empty.
adlist: fix listJoin() in the case the second list is empty.

See #4192, the original PR removed lines of code that are actually
needed, so thanks to @chunqiulfq for reporting the problem, but merging
solution from @jeesyn after checking, together with @artix75, that the
logic covers all the cases.

soveran/redis,yossigo/redis,PKRoma/redis,oranagra/redis,neomantra/redis,charsyam/redis,oranagra/redis,yossigo/redis,ctripcorp/redis,spinlock/redis,rogerchina/redis,spinlock/redis,GitHubMota/redis,pmem/redis,antirez/redis,neomantra/redis,pmem/redis,ofirluzon/redis,pmem/redis,Sciumo/redis,antirez/redis,antirez/redis,yossigo/redis,soveran/redis,ofirluzon/redis,Sciumo/redis,ofirluzon/redis,yossigo/redis,nnog/redis,soloestoy/redis,oranagra/redis,ctripcorp/redis,soveran/redis,ofirluzon/redis,oranagra/redis,neomantra/redis,oranagra/redis,soveran/redis,rogerchina/redis,spinlock/redis,nnog/redis,soloestoy/redis,nnog/redis,Sciumo/redis,soloestoy/redis,neomantra/redis,soloestoy/redis,yossigo/redis,charsyam/redis,rogerchina/redis,GitHubMota/redis,neomantra/redis,ctripcorp/redis,antirez/redis,PKRoma/redis,soloestoy/redis,rogerchina/redis,GitHubMota/redis,spinlock/redis,pmem/redis,PKRoma/redis,charsyam/redis,PKRoma/redis,GitHubMota/redis,ctripcorp/redis,ofirluzon/redis,Sciumo/redis,charsyam/redis,charsyam/redis,nnog/redis,PKRoma/redis