wx simple DateEditor view shows a date despite the underlying value being none

This is an analogous issue to #967. In Qt with a date that is None the default shown in 01/01/2000, whereas on wx it is the current date.

As mentioned on the Qt issue (see the reference link), I am not sure if Qt will be able to accept a None value without some decently ugly workarounds (at least from what I have seen so far). It would be good for the behavior to be the same between Qt and wx (so if we are unable to set Qt to none we may want to change it to be the current date to match wx in which case this is no longer a valid issue, or we may want to do something else entirely)

Also, for wx the docs mention a style option to allow None as a date (we currently already use this):
"wx.adv.DP_ALLOWNONE: With this style, the control allows the user to not enter any valid date at all. Without it - the default - the control always has some valid date. This style is not supported in OSX/Cocoa native version."

I briefly was looking into fixing this, but being that Im on OSX was unsuccessful. I may take a look at it on a linux vm later, but regardless it seems having a None valued date may not be allowed on OSX in which case we may want to handle this differently.