fix(client): Return nil and error if endpoint is empty string

Why:
Passing an empty string as an endpoint to Client when instantiating a
new client might seem like something that should never happen but I
managed to trigger it while parsing some input files to register feeds
in bulk.

What:
In the execute() function, check early if the endpoint is "" and then
return immediately nil and a new error, named ErrEmptyEndpoint with a
descriptive string

0 files changed, 0 insertions, 0 deletions