Actually this was already the case for certain commands, but some of them
was left unguarded for such cases. This commit also fixes#109.
The behaviour of silently skipping key prefixing when a command has no
arguments may change in the future so we added explicit tests as guards
for future changes. Predis\Command\Processor\KeyPrefixProcessor will
continue to skip key prefixing on empty arguments, regardless.
The reason for this change is to support the upcoming redis-cluster since
it has different behaviors compared to the client-side cluster implementation
provided by Predis. For example redis-cluster will not support key tags or
certain operations currently available with our client-side implementation.
Daniele Alessandri