Allow empty lines in commands.
During DIGEST-MD5 authentication the client response with a an empty
line (Just \r\n). eimap_utils:split_command_into_components Either
returns a full command or an empty one, and thus cannot differentiate
between an empty and an incomplete command, and then erroneously tries
to buffer until the rest arrives (which is not existinent).
Ideally we would fix split_command_into_components into returning
something entirely different if the command is empty but complete,
but this fixes the problem at hand by matching for that special case.