fix Trying to access array offset on value of type null (7.4)

[remicollet]

Without this fix

1) Swaggest\JsonSchema\Tests\PHPUnit\ClassStructure\ClassStructureTest::testAdditionalProperties
Trying to access array offset on value of type null

/dev/shm/BUILDROOT/php-swaggest-json-schema-0.12.27-1.fc31.remi.x86_64/usr/share/php/Swaggest/JsonSchema/Wrapper.php:108
/dev/shm/BUILDROOT/php-swaggest-json-schema-0.12.27-1.fc31.remi.x86_64/usr/share/php/Swaggest/JsonSchema/Structure/ClassStructureTrait.php:161
/dev/shm/BUILD/php-json-schema-9ec863a05a40e9b4547bc0c43dc6992cf17307a3/tests/src/PHPUnit/ClassStructure/ClassStructureTest.php:102

2) Swaggest\JsonSchema\Tests\PHPUnit\ClassStructure\ClassStructureTest::testPatternProperties
Trying to access array offset on value of type null

/dev/shm/BUILDROOT/php-swaggest-json-schema-0.12.27-1.fc31.remi.x86_64/usr/share/php/Swaggest/JsonSchema/Wrapper.php:108
/dev/shm/BUILDROOT/php-swaggest-json-schema-0.12.27-1.fc31.remi.x86_64/usr/share/php/Swaggest/JsonSchema/Structure/ClassStructureTrait.php:161
/dev/shm/BUILD/php-json-schema-9ec863a05a40e9b4547bc0c43dc6992cf17307a3/tests/src/Helper/SampleProperties.php:43
/dev/shm/BUILD/php-json-schema-9ec863a05a40e9b4547bc0c43dc6992cf17307a3/tests/src/PHPUnit/ClassStructure/ClassStructureTest.php:122

[codecov]

Codecov Report

Merging #101 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
+ Coverage   92.15%   92.27%   +0.12%     
==========================================
  Files          27       27              
  Lines        1619     1619              
==========================================
+ Hits         1492     1494       +2     
+ Misses        127      125       -2

Impacted Files	Coverage Δ
src/Wrapper.php	57.62% <100%> (ø)	⬆️
src/MagicMapTrait.php	83.78% <0%> (+5.4%)	⬆️

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[vearutop]

Thank you, tagged as v0.12.28.