Skip Explores without dimensions

[joshtemple]

Change description

This change modifies Spectacles SQL validation behavior from raising an exception on Explores with zero dimensions to instead logging a warning and marking the Explore as skipped.

Here's an example of the output. Note the warning log and the improvement in the "Testing" header, as well as eye_exam.users__no_dims being marked as skipped with a reason for skipping.

(spectacles) ➜ spectacles sql --project eye_exam --branch dev-josh-temple-8m75 --incremental
Connected to Looker version 23.6.53 using Looker API 4.0
Warning: Explore 'users__no_dims' does not have any non-ignored dimensions and will not be validated.

=================== Testing 2/5 explores [concurrency = 10] ====================

✓ eye_exam.users passed
✓ eye_exam.users__fail skipped (unmodified)
✓ eye_exam.users__fail_and_warn skipped (unmodified)
✓ eye_exam.users__no_dims skipped (no dimensions)
✓ eye_exam.users__warn passed

Type of change

• Bug fix (fixes an issue)
• New feature (adds functionality)
• Improvement

Checklists

Security

• Security impact of change has been considered
• Code follows security best practices and guidelines

Code review

• Pull request has a descriptive title and context useful to a reviewer