Fix: Do not attempt to fetch and modify properties of something that isn't an object

[localheinz]

This PR

• asserts that normalizers skip normalization when json_decode()ed data is not an object
• skips normalization when json_decode()ed data is not an object

[codecov]

Codecov Report

Merging #87 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master    #87   +/-   ##
=======================================
  Coverage       100%   100%           
- Complexity       69     74    +5     
=======================================
  Files             7      7           
  Lines           264    275   +11     
=======================================
+ Hits            264    275   +11

Impacted Files	Coverage Δ	Complexity Δ
src/Normalizer/VersionConstraintNormalizer.php	100% <100%> (ø)	10 <0> (+1)	⬆️
src/Normalizer/ConfigHashNormalizer.php	100% <100%> (ø)	7 <0> (+1)	⬆️
src/Normalizer/PackageHashNormalizer.php	100% <100%> (ø)	10 <0> (+1)	⬆️
src/Normalizer/BinNormalizer.php	100% <100%> (ø)	6 <0> (+1)	⬆️
src/Normalizer/ComposerJsonNormalizer.php	100% <100%> (ø)	5 <0> (+1)	⬆️

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