Fix NoMethodError in issuable import CSV base service

What does this MR do?

Related to #246857 (closed)

In !46361 (merged) we refactored Issues::ImportCsvService and moved its content to Issuable::ImportCsv::BaseService so it could be reused to import Requirements.

In that process, we added a method to check for the presence of headers in the CSV file but missed the scenario of an empty file that would result in a NoMethodError as following:

Failures:
  1) RequirementsManagement::ImportRequirementsCsvWorker#perform behaves like an idempotent worker performs multiple times sequentially without raising an exception
     Failure/Error: expect { subject }.not_to raise_error
       expected no Exception, got #<NoMethodError: undefined method `downcase' for nil:NilClass> with backtrace:
         # ./app/services/issuable/import_csv/base_service.rb:53:in `verify_headers!'
         # ./app/services/issuable/import_csv/base_service.rb:41:in `with_csv_lines'
         # ./app/services/issuable/import_csv/base_service.rb:23:in `process_csv'
         # ./app/services/issuable/import_csv/base_service.rb:14:in `execute'

This MR adds an extra step that checks for the presence of lines in the CSV data and a new test case for empty files.

Screenshots (strongly suggested)

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

Related to #246857 (closed)