fix: preserve original scalar text for null nodes so as<string> returns the source text

[mvanhorn]

Summary

Plumb the original token text through the null path without breaking the public EventHandler API: add a non-pure virtual OnNull(const Mark&, anchor_t, const std::string& value) overload to include/yaml-cpp/eventhandler.h whose default implementation delegates to the existing two-arg OnNull, so third-party handlers and the emitter/graphbuilder adapters keep working unchanged. SingleDocParser::HandleNode passes token.value at the null-plain-scalar site; NodeBuilder overrides the three-arg OnNull and calls a new node_data::set_null(const std::string& scalar) overload (include/yaml-cpp/node/detail/node_data.h + src/node_data.cpp) that sets m_type = NodeType::Null while storing the source text in m_scalar, exposed via the existing Scalar() accessor. Finally, the two as_if<std::string> specializations in include/yaml-cpp/node/impl.h return node.Scalar() for Null nodes instead of the literal "null", so key: yields "", key: ~ yields "~", and key: null yields "null", while IsNull() stays true for all of them.

Why this matters

Since yaml-cpp 0.8.0, any null-like plain scalar (<empty>, ~, null, Null, NULL) loses its original text: SingleDocParser::HandleNode (src/singledocparser.cpp, ~line 101) detects IsNullString(token.value...) and emits OnNull(mark, anchor), discarding token.value. NodeBuilder::OnNull then calls node.set_null(), which leaves m_scalar empty, and as_if<std::string> in include/yaml-cpp/node/impl.h (~lines 119 and 151) hard-codes the return value "null" for NodeType::Null. The reporter cannot distinguish message: (empty) from message: null because both Scalar() and as<std::string>() return fabricated values. Owner jbeder agreed the original text representation should be kept in Scalar() and returned by as<std::string>() ("Yeah that's reasonable").

See #1290.

Testing

• Happy path: load message: null and assert IsNull() is true, Scalar() == "null", and as<std::string>() == "null" (test/integration/load_node_test.cpp). - Variants: key: yields "", key: ~ yields "~", key: Null / key: NULL yield their exact source text; quoted "null" remains a plain string scalar unaffected. - Edge cases: flow-map and block-seq missing values (structural nulls with no token) return "" from as<std::string>() and stay IsNull(); a default-constructed Node() still reports Null with empty scalar (test/node/node_test.cpp). - Error paths: as<std::string>() on a map/sequence still throws TypedBadConversion; as<std::string>(fallback) on non-scalar non-null still returns the fallback.

Fixes #1290

[mvanhorn]

Addressed in 30eade1. Full CTest suite (1029 tests) passes locally.