check if certificates is empty strings

[e-max]

Kafka driver enable TLS connection if user passed key and certificate. Before we consider any certificate valid if it wasn't nil. In this PR we check if it also not empty string

[dgellow]

@e-max What about using a string instead of a []byte, that we then only cast when needed (i.e: just before the call to newTLSConnection, in the case where we want to use TLS)?

[dgellow]

That way the caller doesn't have to cast anything and can directly pass a value from os.Getenv.

[dgellow]

Also, do we need tests for this change?

[dgellow]

Why is the update of certificates in tests necessary?

Edit, based on slack exchange:

tests were failing because our tests certificates had expired. I've updated them in the same branch

[e-max]

arr. Guthub stopped to send me an email again! I missed all your comments @dgellow . Sorry about it.