fix: avoid panic by checking response for nil

[Zebradil]

Fixes #389.

This PR addresses two issues:

• panic in case of malformed API token
• accidental trailing spaces in the token value

If the authorization header value contains a trailing newline, the http client returns error without a response struct (nil).

It is easy to miss a trailing newline when encoding a value into base64. To avoid inconvenience for users, let's trim it.

[joaopapereira]

LGTM