Make IndexService::find() adhere to its contract when document was not found

[jdreesen]

Elasticsearch\Client::get() throws an exception when the document is not found but according to its PhpDoc ONGR\ElasticsearchBundle\Service\IndexService::find() should return null in this case.

[codecov-io]

Codecov Report

Merging #971 into 6.2 will decrease coverage by 0.09%.
The diff coverage is 33.33%.

@@             Coverage Diff              @@
##                6.2     #971      +/-   ##
============================================
- Coverage     66.61%   66.52%   -0.10%     
- Complexity      423      424       +1     
============================================
  Files            39       39              
  Lines          1393     1395       +2     
============================================
  Hits            928      928              
- Misses          465      467       +2     

Impacted Files	Coverage Δ	Complexity Δ
Service/IndexService.php	63.01% <33.33%> (-0.59%)	66.00 <0.00> (+1.00)	⬇️

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