Update message about no published lessons to be more clear

[renatho]

Proposed Changes

• Improve frontend message when there's no published lessons yet.

Testing Instructions

1. Create a course.
2. Publish it without publishing the lessons.
3. Visit the course on the frontend and make sure the message was changed.

Screenshot

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[codecov]

Codecov Report

Merging #7300 (1120348) into trunk (1baf3db) will increase coverage by 0.00%.
Report is 105 commits behind head on trunk.
The diff coverage is 69.47%.

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk    #7300   +/-   ##
=========================================
  Coverage     50.81%   50.81%           
- Complexity    11053    11058    +5     
=========================================
  Files           611      611           
  Lines         46721    46725    +4     
  Branches        404      404           
=========================================
+ Hits          23740    23743    +3     
- Misses        22654    22655    +1     
  Partials        327      327           

Files	Coverage Δ
...locks/class-sensei-course-outline-course-block.php	80.55% <100.00%> (ø)
...ludes/blocks/course-theme/class-lesson-actions.php	98.00% <100.00%> (ø)
includes/class-sensei-feature-flags.php	96.77% <ø> (ø)
includes/class-sensei-learner.php	42.23% <100.00%> (ø)
includes/class-sensei-notices.php	50.76% <100.00%> (ø)
includes/class-sensei-quiz.php	69.45% <100.00%> (ø)
includes/class-sensei-utils.php	52.30% <100.00%> (+0.08%)	⬆️
.../course-theme/class-sensei-course-theme-lesson.php	94.81% <100.00%> (ø)
includes/internal/installer/class-schema.php	100.00% <100.00%> (ø)
.../class-comments-based-quiz-progress-repository.php	60.86% <50.00%> (-0.16%)	⬇️
... and 6 more

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