Update ee/lib/tasks/gitlab/license.rake to check if a license exists rather than crashing with an error

What does this MR do?

When running the gitlab-rake gitlab:license:info maintenance task on a GitLab instance which does not have an applied license, the license info method will crash with the following error:

Today's Date: 2020-08-10
Current User Count: 7
Max Historical Count: 
Max Users in License: 
License valid from:  to 
rake aborted!
NoMethodError: undefined method `[]' for nil:NilClass
/opt/gitlab/embedded/service/gitlab-rails/ee/lib/tasks/gitlab/license.rake:13:in `block (3 levels) in <top (required)>'
/opt/gitlab/embedded/bin/bundle:23:in `load'
/opt/gitlab/embedded/bin/bundle:23:in `<main>'
Tasks: TOP => gitlab:license:info
(See full trace by running task with --trace)

This happens because the method is trying to print the Email attribute which does not exist in the license object in an unlicensed GitLab instance.

This MR changes that functionality by inserting a condition that checks for the presence of the license_plan attribute before attempting any of the puts which provide the output of this method. If no license_plan attribute exists, then the conditional will print a No license has been applied. message instead.

I selected the license_plan attribute because it seemed to fit more with the intent of the check, despite the actual failure happening when the Email attribute was being fetched.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team