Nicholas of Myra never was at Nicea. He is more imaginary than Godric Gryffindor.
“The Eastern bishops formed the great majority. Of these, the first rank was held by the three patriarchs: Alexander of Alexandria, Eustathius of Antioch, and Macarius of Jerusalem. Many of the assembled fathers—for instance, Paphnutius of Thebes, Potamon of Heraclea and Paul of Neocaesarea—had stood forth as confessors of the faith and came to the council with the marks of persecution on their faces. This position is supported by patristic scholar Timothy Barnes in his book Constantine and Eusebius.[27] Historically, the influence of these marred confessors has been seen as substantial, but recent scholarship has called this into question.[25]
Other remarkable attendees were Eusebius of Nicomedia; Eusebius of Caesarea, the purported first church historian; circumstances suggest that Nicholas of Myra attended (his life was the seed of the Santa Claus legends); Aristakes of Armenia (son of Saint Gregory the Illuminator); Leontius of Caesarea; Jacob of Nisibis, a former hermit; Hypatius of Gangra; Protogenes of Sardica; Melitius of Sebastopolis; Achilleus of Larissa (considered the Athanasius of Thessaly)[28] and Spyridion of Trimythous, who even while a bishop made his living as a shepherd[29] From foreign places came John, bishop of Persia and India, Theophilus, a Gothic bishop and Stratophilus, bishop of Pitiunt of Georgia.”
http://en.wikipedia.org/wiki/First_Council_of_Nicaea#CITEREFAtiya1991
Doubting his existence requires you doubt the existence of a lot of historical figures, including many of those listed above.