Fix parsing input which begins with a newline & indentation

[fktn-k]

This PR fixes parse error in building example files for basic_node::deserialize() function.
The root cause was that #432 introduced a bug in parsing an input which

• begins with a newline and
• contains a plain scalar as a mapping value with indentation on the second line

The above two conditions caused invalid indentation detections.
This bug can be reproduced by running this cpp file:

#include <fkYAML/node.hpp>
int main() {
    std::string input = R"(
    foo: true
    bar: 123
    baz: 3.14
    )";
    return 0;
}

The above snipet now runs as expected by fixing detection of indentation during deserialization.

Moreover, some minor fixes have been made to resolve warnings during example builds.

---

Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• Changes are described in the pull request or in a referenced issue.
• The test suite compiles and runs without any error.
• The code coverage on your branch is 100%.
• The documentation is updated if you added/changed a feature.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Please refrain from proposing changes that would break YAML specifications. If you propose a conformant extension of YAML to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[github-actions]

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