Issue #3257654 by mondrake: Fix PHPStan L0 failures that cannot be included in baseline

core/lib/Drupal/Core/File/MimeType/MimeTypeGuesser.php

core/lib/Drupal/Core/TypedData/ComputedItemListTrait.php

core/modules/comment/src/CommentFieldItemList.php

.../migrate/src/Plugin/migrate/source/EmbeddedDataSource.php

...src/Plugin/migrate/source/CacheableEmbeddedDataSource.php

...igrate_drupal/src/Plugin/migrate/source/ContentEntity.php

...tests/Drupal/FunctionalTests/Bootstrap/ErrorContainer.php

.../Drupal/Tests/Component/ProxyBuilder/ProxyBuilderTest.php

...ts/Core/DependencyInjection/Compiler/MimeTypePassTest.php