internal: fix always-true interface comparison

[sbinet]

This CL corrects the following bug uncovered by staticcheck:

  internal/elements.go:148:6: this comparison is always true (SA4023)
    internal/elements.go:146:18: the lhs of the comparison gets its value from here and has a concrete type

[emersion]

Ouch. That's the usual "nil pointer but not actually nil interface" footgun right?

Can we ensure that we never return a nil *Error (which ends up being a non-nil error)? Even if resp.ResponseDescription == ""?

Probably this would help:

var err error
if resp.Error != nil {
    err = resp.Error
}

[sbinet]

That's the usual "nil pointer but not actually nil interface" footgun right?

yep, that's the one.

PTAL.

[emersion]

Thank you!