error in setupTickGeneration when axis.delta = 0 #1869

Version: flot 4.2.6

In setupTickGeneration, if axis.delta=0 (max and min are the same) then an error is thrown because we cannot call toFixed(Infinity)

https://github.com/flot/flot/blob/84f3e5d5e997f2f7c0180314ccbc6175ad40cab7/source/jquery.flot.js#L1657C1-L1669C22