fix: show "This <containerType> is empty" [FC-0090]

[pomegranited]

Description

Shows "This is empty" text when container child list is empty for units, subsections, and sections.

Affects Content Library Authors.

Supporting information

Addresses comments:

• Subsection full-page view (barebones) #1613 (comment)
• Section full-page view (barebones) #1614 (comment)

Private-ref: FAL-4191

Testing instructions

1. In a content library, create a new empty Unit, Subsection, and Section.
2. From the main library page, click through each of the empty container cards and check that expected text displays in the sidebar preview, e.g "This unit is empty
3. Open each empty container -- empty text from the sidebar preview above should display on the page.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.07%. Comparing base (eaee525) to head (c643e8e).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2157      +/-   ##
==========================================
- Coverage   94.07%   94.07%   -0.01%     
==========================================
  Files        1164     1164              
  Lines       24473    24477       +4     
  Branches     5320     5303      -17     
==========================================
+ Hits        23023    23026       +3     
- Misses       1373     1374       +1     
  Partials       77       77              

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[ChrisChV]

Looks good! 👍

• I tested this: I followed the testing instructions and verified that the tests are gone
• I read through the code and considered the security, stability and performance implications of the changes.
• I tested that the UI can be used with a keyboard only (tab order, keyboard controls).
• Includes tests for bugfixes and/or features added.
• Includes documentation