Fix: Checking if yerr==None before using it.

[RHofsaess]

If yerr is None, zip() does not work and throws an error.
Therefore, it has to be checked before.

Note: with this implementation, the whole step ist skipped! I am not sure, if this is generally intended.
But just calling:

if _yerr is None:
    _yerr = 0, 0

Also doesn't do the trick, as the data type and the dimensions do not match!