Fix issues with register_tasks and empty tasks list

[nazywam]

Closes #236

When backend.register_tasks -

karton/karton/core/backend.py

Line 542 in 2d10bd4

def register_tasks(self, tasks: List[Task]) -> None:

is called with an empty list of tasks to create it results in a crash when calling mset:

redis.exceptions.ResponseError: wrong number of arguments for 'mset' command

Reproduction:
Downgrade to <4.4.0, create a new task and create a new "NOP" operational task by setting the task status to its current value:

producer = Producer(identity="test-producer")
headers = {
    "kind": "raw",
    "type": "sample"
}
t = Task(headers=headers, payload={
    "sample": "sample",
})
producer.send_task(t)
producer.backend.set_task_status(t, TaskState.DECLARED)