So did I. It just pointed out the error while accepting my answer though.
Just by looking at the quotes from various sites listed in a Google search, one gets the impression that both spellings are used by Welsh writers, but that the spelling without the c is more common. I suggest that the problem is that Welsh speakers, both in Welsh and in English, do not usually pronounce the c. So, in order to keep the Welsh spelling as phonetic as possible, i.e. so that there are no silent letters in the spelling, it has become the custom to spell this word without the c. Maybe someone could confirm or refute my suggestion?