Fix user reference transformation in EpicsPipeline

What does this MR do?

This MR fixes a bug in EE::BulkImports::Groups::Pipelines::EpicsPipeline where user reference wasn't properly updated and exception was raised { "exception_class":"ActiveRecord::AssociationTypeMismatch","exception_message":"User(#89020) expected, got {\"public_email\"=\u003e\"\"} which is an instance of Hash(#23720)"} because UserReferenceTransformer expects custom reference to be a string, not a symbol.

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