fix conversion of DateTime to string in getContainsHtml

[osma]

Reasons for creating this PR

There is a recently surfaced (probably PHP 8.x related) problem in the YSO-aika (YSO Time) vocabulary. Concept pages failed to display and instead we got this message in the error log:

[Wed Mar 22 09:53:30.124150 2023] [php:error] [pid 709825] [client 127.0.0.1:38096] PHP Fatal error:  Uncaught TypeError: preg_match(): Argument #2 ($subject) must be of type string, DateTime given in /var/www/html/Skosmos/model/ConceptPropertyValueLiteral.php:60

The reason is pretty simple: if a concept has a property with a datetime value, it cannot be implicitly casted to a string value. This PR fixes the problem by relying on the getLabel() method in the same class, which already handles DateTime to string conversion.

Link to relevant issue(s), if any

(not reported as an issue, I decided to fix this directly)

Description of the changes in this PR

• fix DateTime to string conversion in ConceptPropertyValueLiteral.getContainsHtml() by relying on .getLabel()
• add unit test for verifying above conversion

Known problems or uncertainties in this PR

none

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (a59f251) 69.55% compared to head (bc490ba) 69.55%.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1423   +/-   ##
=========================================
  Coverage     69.55%   69.55%           
  Complexity     1650     1650           
=========================================
  Files            32       32           
  Lines          4257     4257           
=========================================
  Hits           2961     2961           
  Misses         1296     1296           

Impacted Files	Coverage Δ
model/ConceptPropertyValueLiteral.php	95.55% <100.00%> (ø)

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[miguelvaara]

Great job @osma 👍