NIFI-3861: Fixed AvroReader nullable logical types issue

[ijokarumawak]

• AvroReader did not convert logical types if those are defined with union
• Consolidated createSchema method in AvroSchemaRegistry and AvroTypeUtil as both has identical implementation and mai
  ntaining both would be error-prone

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[markap14]

Reviewing...

[markap14]

All looks good. Thanks for catching this one too, @ijokarumawak! +1 merged to master.