Previously we would ignore unsetting calls to MakeCurrent. But this
would prevent ANGLE from unsetting driver state on the current thread.

Fix this by checking for 0/null explicitly in DisplayWGL::makeCurrent.

This turned up when running test configs without recreating the
Display.

Bug: angleproject:3261
Change-Id: I7f747dc7649a06f019f1b89655cb71dd3b151aa0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1520992
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>