Refactor the generateSampleValue method to avoid saving data and fit with core...

Mathieu Le Cainrequested to merge
issue/entity_reference_revisions-2568187:2568187-align-generatesamplevalue-with-core-entityref into 8.x-1.x

Refactor the generateSampleValue method to avoid saving data and fit with core EntityReferenceItem strategy

Closes #2568187

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