Fix conversions from empty tuples

[fktn-k]

This PR fixes wrong conversions from std::tuple<> to basic_node.
That was introduced in the PR #465.
std::tuple<> was converted to a null node while an empty sequence was (and still is) converted to std::tuple<>.
Now, std::tuple<> and an empty sequence are guranteed to be converted to each other.

// BEFORE the fix
std::tuple<> t {};
fkyaml::node n = t; // `n` used to be converted as a null scalar node.

// AFTER the fix
fkyaml::node n = t; // `n` is now converted as an empty sequence node.

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