ebai solution plotting: lack of sigmas causes error in automatic ylim

[kecnry]

See thread in https://github.com/phoebe-project/phoebe2/discussions/462#discussioncomment-847040_ for details

When the dataset does not contain sigmas, those are passed on to plotting as nans which is tripping the attempt to automatically determine ylimits. This should be easily avoided by not passing the sigmas if all nans (not provided in the original dataset).