Fix a panic in the Redis processor

[mihaitodor]

Ensure that the Redis processor has the args_mapping field. Fixes #1371.

It now outputs the following:

ERRO Config lint error                             @service=benthos lint="./tmp/config_redis_cache.yaml: line 7: field args_mapping is required"
ERRO Shutting down due to linter errors, to prevent shutdown run Benthos with --chilled  @service=benthos