Fix an issue in the create_empty_index rake task

What does this MR do?

This MR is to fix a recent introduced issue in create_empty_index rake task.

Screenshots (strongly suggested)

Before

> bundle exec rake gitlab:elastic:create_empty_index RAILS_ENV=development

NameError: undefined local variable or method `migrations_index_name' for main:Object
Did you mean?  migration_running?
/Users/cliu/gitlab/gitlab-development-kit/gitlab/ee/lib/tasks/gitlab/elastic.rake:71:in `block (3 levels) in <main>'
/Users/cliu/.rbenv/versions/2.7.2/bin/bundle:23:in `load'
/Users/cliu/.rbenv/versions/2.7.2/bin/bundle:23:in `<main>'
Tasks: TOP => gitlab:elastic:create_empty_index
(See full trace by running task with --trace)

After

> bundle exec rake gitlab:elastic:create_empty_index RAILS_ENV=development

Index 'gitlab-development-20201112-1316' has been created.
Alias 'gitlab-development' → 'gitlab-development-20201112-1316' has been created

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