Fix issues reported by SonarCloud

[osma]

This PR should resolve all the "Bugs" and some of the "Code Smells" reported by SonarCloud. Fixes #916

There's more to be done in this area, but at least this PR takes care of the low hanging fruit.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
4 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #924 into master will increase coverage by 0.09%.
The diff coverage is 56.25%.

@@             Coverage Diff              @@
##             master     #924      +/-   ##
============================================
+ Coverage     57.68%   57.78%   +0.09%     
  Complexity     1489     1489              
============================================
  Files            32       32              
  Lines          4178     4164      -14     
============================================
- Hits           2410     2406       -4     
+ Misses         1768     1758      -10

Impacted Files	Coverage Δ	Complexity Δ
controller/Controller.php	13.48% <0%> (+0.29%)	38 <0> (ø)	⬇️
model/Model.php	80.32% <0%> (+0.32%)	105 <0> (ø)	⬇️
controller/RestController.php	3.99% <0%> (+0.01%)	147 <0> (ø)	⬇️
controller/WebController.php	17.75% <0%> (+0.04%)	95 <0> (ø)	⬇️
model/ConceptSearchParameters.php	84.26% <100%> (-0.18%)	58 <0> (ø)
controller/Honeypot.php	38.46% <100%> (-2.28%)	11 <0> (ø)
model/ConceptMappingPropertyValue.php	72.32% <100%> (-0.49%)	41 <0> (ø)
model/ConceptPropertyValue.php	79.26% <100%> (-0.25%)	46 <0> (ø)
model/Concept.php	79.31% <66.66%> (+0.09%)	184 <0> (ø)	⬇️
model/resolver/LinkedDataResource.php	93.33% <0%> (+20%)	3% <0%> (ø)	⬇️

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