Don't specify default value for FactorRange.{start,end}

[mattpap]

Prior to PR #12565 all readonly properties had an intrinsic default value, which usually doesn't make sense. So now they don't except for this, because FactorRange in bokehjs sets start and end during model initialization, which always results in 0, because that happens before sensible values can be computed. Due to how defaults' testing works, this requires setting the default value in bokeh. Ultimately this default will be removed, which will require changes to FactorRange's initialization, which I don't want to get into here.

Originally posted by @mattpap in #12565 (comment)