gl::GetString check for a valid context 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/295142 Tested-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>