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 <hepler@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>

pw_span/public/pw_span/internal/span.h

@@ -262,6 +262,9 @@ class span : public pw_span_internal::ExtentStorage<Extent> {
     _PW_SPAN_ASSERT(Extent == dynamic_extent || Extent == size);
   }
 
+  // Prevent construction from nullptr, which is disallowed by C++20's std::span
+  constexpr span(std::nullptr_t data, size_t size) = delete;
+
   // Artificially templatized to break ambiguity for span(ptr, 0).
   template <typename = void>
   constexpr span(T* begin, T* end) noexcept : span(begin, end - begin) {