fix(src): avoid possible infinite loop in LoadAll().

[FedeDP]

Leave at first empty root.

This avoids an infinite loop in case of HandleNextDocument returning true even if the document has errors, basically because no token.type gets matched in SingleDocParser::HandleNode, leading to eventHandler.OnNull(mark, anchor); being called indefinitely at each iteration, pushing a null node to documents vector.

Another solution would be to add a m_scanner.pop(); in the default switch case in SingleDocParser::HandleNode to make sure we always consume the current token.

The behavior causes this issue: https://github.com/falcosecurity/falco/issues/3281

Easily reproducible with a simple c++ example:

#include <yaml-cpp/yaml.h>

int main() {
    static const std::string yml = R"(
,
    )";
    YAML::LoadAll(yml);
    return 0;
}