fix build: fix MonotonicConcurrentSet kNullTable being non-constexpr-able

commit_hash:2858f97bbcd9b497a6e6544557c760589325be72

Author: Anton3

universal/include/userver/utils/span.hpp

@@ -35,11 +35,16 @@ class span final {  // NOLINT(readability-identifier-naming)
     constexpr span() noexcept : span(nullptr, nullptr) {}
 
     constexpr span(T* begin, T* end) noexcept : begin_(begin), end_(end) {
-        UASSERT((begin != nullptr && end != nullptr && begin <= end) || (begin == nullptr && end == nullptr));
+        // GCC 11.4.0 has issues with comparing pointers at compile time.
+        UASSERT(
+            std::is_constant_evaluated() || (begin != nullptr && end != nullptr && begin <= end) ||
+            (begin == nullptr && end == nullptr)
+        );
     }
 
     constexpr span(T* begin, std::size_t size) noexcept : begin_(begin), end_(begin + size) {
-        UASSERT(begin != nullptr || size == 0);
+        // GCC 11.4.0 has issues with comparing pointers at compile time.
+        UASSERT(std::is_constant_evaluated() || begin != nullptr || size == 0);
     }
 
 #if defined(__GNUC__) && !defined(__clang__)