TST: added null conversions for TimeDeltaBlock in core/internals related (GH4396)
closes #4396

NaT wraparound bug again, fixed

cc @yarikoptic

I am pretty sure this fixes the open sparc issues. Except for 1 thing.

We are doing a test for clipboard access, which uses xclip. But since there isn't a display attached this (and the plotting tests) fail. I am not sure that we can catch this. 

we test using a virtual frame buffer, can you test doing something like (need to install X)

```
export DISPLAY=:99.0
/etc/init.d/xvfb start
```
