For some reason, 7f10d1fa8a handled
wxSYS_CARET_ON_MSEC and wxSYS_CARET_OFF_MSEC differently in wxGTK, even
though it looks like they should be handled in exactly the same way.
Change the code to do this, which results in returning 0 for the former
setting if the caret doesn't blink at all, just as was already the case
for the latter one.
See https://github.com/wxWidgets/wxWidgets/pull/1285
See #17629.