glGetString() without a current context could cause a null dereference
for some enums. Always return nullptr when no context is bound.

BUG=angleproject:1106

Change-Id: Ic36f1adff8b2e3cd54a7b33b2e12899781feba82
Reviewed-on: https://chromium-review.googlesource.com/295142Tested-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>