pw_span: Disallow construction from std::nullptr_t
pw_span: Disallow construction from std::nullptr_t

C++20's std::span does not allow constructing spans from std::nullptr_t.
Disallow that in Pigweed's span for forward compatibility.

Change-Id: I99556a982f952b7fb5f64360b3a230838925ea72
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/96562
Pigweed-Auto-Submit: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Reviewed-by: Armando Montanez <97ef6fdb231c129ac8827d9f0c4106bfbf57f74f@google.com>
Commit-Queue: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed