How does it have yna and yma in the same sentence? What does the 'na add ?
Compare the English sentence: "There is nothing here".
It has both a "there" in it and a "here".
The word "there" has no literal meaning; "there is" merely indicates the existence of something but not its location -- the location is specified by the word "here".
The Welsh construction seems to work similarly, with (d)oes 'na for talking about the (non-)existence and yma specifying the location where the thing exists (or does not exist).