Empty `std::tuple<>` cannot be serialized and de-serialized

[moratom]

Description

The behavior of serializing and de-sereailzing an empty tuple changed from 3.9.1 to 3.10.0.

null used to de-serialize without a problem to std::tuple<> but since throws an error since 3.10.0.

The change of behavior happened in #2576.

Note that I'm not entirely sure this is a bug per-say, but we relied on this behavior in https://github.com/luxonis/depthai-core so I decided to report in case the previous behavior was "more correct".

Reproduction steps

To reproduce run the MRE with both 3.10.0 and 3.9.1.

Expected vs. actual results

Expected - null succesfully de-serializes when j.get<std::tuple<>>() is called on null.
Actual:

terminate called after throwing an instance of 'nlohmann::detail::type_error'
  what():  [json.exception.type_error.302] type must be array, but is null

Minimal code example

#include <nlohmann/json.hpp>

int main() {
    auto sourceTuple = std::tuple<>();
    nlohmann::json j = sourceTuple;
    auto returnedTuple = j.get<std::tuple<>>();
    return 0;
}

Error messages

terminate called after throwing an instance of 'nlohmann::detail::type_error'
  what():  [json.exception.type_error.302] type must be array, but is null
Aborted (core dumped)

Compiler and operating system

Ubuntu 20.04, GCC 9.4.0

Library version

3.11.3

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.

[codenut]

This bug occurs because an empty tuple std::tuple<>() is being serialized as "null" instead of "[]".

Current behavior

nlohmann::json j = sourceTuple;  
auto js = j.dump(); // "null"

Expected behavior

nlohmann::json j = sourceTuple;  
auto js = j.dump(); // "[]"

@nlohmann I will work on a fix for this.

[nlohmann]

As discussed in #4594 (comment), the expected behavior for an empty tuple is to be serialized to [] and from there back to an empty tuple. The expectation to deserialize null to an empty tuple is wrong, as other types such as strings or numbers also do not deserialize from null to any value.