Fix: Avoid use of isset()

[localheinz]

This PR

• avoids the use of isset()

Follows #1.

[codecov]

Codecov Report

Merging #74 into master will not change coverage by %.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master      #74   +/-   ##
=========================================
  Coverage     93.40%   93.40%           
  Complexity       77       77           
=========================================
  Files             8        8           
  Lines           197      197           
=========================================
  Hits            184      184           
  Misses           13       13           

Impacted Files	Coverage Δ	Complexity Δ
src/EntityDef.php	94.73% <100.00%> (ø)	14.00 <0.00> (ø)
src/FixtureFactory.php	91.11% <100.00%> (ø)	37.00 <0.00> (ø)

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