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.