Fix: Reject invalid field definition values

[localheinz]

This PR

• rejects invalid field definition values from the constructor of EntityDefinition

[codecov]

Codecov Report

Merging #142 into master will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #142      +/-   ##
============================================
+ Coverage     98.01%   98.08%   +0.06%     
- Complexity       72       74       +2     
============================================
  Files            12       13       +1     
  Lines           202      209       +7     
============================================
+ Hits            198      205       +7     
  Misses            4        4              

Impacted Files	Coverage Δ	Complexity Δ
src/EntityDefinition.php	100.00% <100.00%> (ø)	5.00 <0.00> (+1.00)
src/Exception/InvalidFieldDefinitions.php	100.00% <100.00%> (ø)	1.00 <1.00> (?)

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