Fix notation data type tests

[osma]

A couple of minor fixes after merging PR #1198:

• fix the coverage annotations (so that the test coverage is properly assessed for the getDatatype method)
• change getDatatype to return null if a datatype exists without a label (and modify corresponding test)

ping @miguelahonen

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #1212 (15b1300) into master (c960e09) will increase coverage by 0.22%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1212      +/-   ##
============================================
+ Coverage     68.13%   68.36%   +0.22%     
  Complexity     1619     1619              
============================================
  Files            32       32              
  Lines          3973     3973              
============================================
+ Hits           2707     2716       +9     
+ Misses         1266     1257       -9     

Impacted Files	Coverage Δ
model/ConceptPropertyValueLiteral.php	70.00% <100.00%> (+18.00%)	⬆️

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[osma]

Hooray, coverage increased by 9 lines! (+0.22%)
Will merge.