Stop the noexcept null constructor delegating to a throwing one

basic_json(std::nullptr_t) delegated to basic_json(value_t), whose
underlying json_value(value_t) constructor allocates for other types
and can therefore throw, which is why the noexcept had a
NOLINT(bugprone-exception-escape). The delegated-to constructor also
called assert_invariant() a second time. The default member
initializers of data already produce the same null state (a
value-initialized, i.e. zeroed, union with object == nullptr), so the
delegation and its NOLINT can simply be dropped.

Part of #5724

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/json.hpp

@@ -1578,8 +1578,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief create a null object
     /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
-    basic_json(std::nullptr_t = nullptr) noexcept // NOLINT(bugprone-exception-escape)
-        : basic_json(value_t::null)
+    basic_json(std::nullptr_t = nullptr) noexcept
     {
         assert_invariant();
     }

single_include/nlohmann/json.hpp

@@ -27659,8 +27659,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief create a null object
     /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
-    basic_json(std::nullptr_t = nullptr) noexcept // NOLINT(bugprone-exception-escape)
-        : basic_json(value_t::null)
+    basic_json(std::nullptr_t = nullptr) noexcept
     {
         assert_invariant();
     }