Allow token to be empty if graphql-token is provided

[jordandcarter]

Description

Fixes tool sign when using --graphql-token.

Context

Running tool sign --graphql-endpoint http://graphql.buildkite.localhost/v1 --graphql-token this_is_not_the_actual_token kept returning an error fatal: Error signing pipeline: couldn't retrieve pipeline: Post "http://graphql.buildkite.localhost/v1": Invalid token, empty string supplied

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go fmt ./...)