Fix crash on no translation in entity_translation producer

[OleksiiTsulun]

Fixes bug when entity_translation data producer tries to get entity's translation without checking if translation exists.

[codecov]

Codecov Report

Merging #1015 into 8.x-4.x will decrease coverage by 0.31%.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##             8.x-4.x    #1015      +/-   ##
=============================================
- Coverage      54.74%   54.43%   -0.32%     
- Complexity       626      627       +1     
=============================================
  Files            113      113              
  Lines           1569     1578       +9     
=============================================
  Hits             859      859              
- Misses           710      719       +9     

Impacted Files	Coverage Δ	Complexity Δ
.../GraphQL/DataProducer/Entity/EntityTranslation.php	60.00% <100.00%> (ø)	9.00 <0.00> (+1.00)
src/Plugin/GraphQL/Schema/SdlSchemaPluginBase.php	41.50% <0.00%> (-4.33%)	19.00% <0.00%> (ø%)
...n/GraphQL/DataProducer/EntityDefinition/Fields.php	0.00% <0.00%> (ø)	12.00% <0.00%> (ø%)
...L/SchemaExtension/SdlSchemaExtensionPluginBase.php	0.00% <0.00%> (ø)	5.00% <0.00%> (ø%)

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[klausi]

Thanks, same as in #1016 : let's add a test case in EntityTest.php. We could also spin out all the entity language tests into their own test class to not make that one too long.