Fix parse error on the first node which isn't a block mapping

[fktn-k]

fkYAML in the latest develop branch emits a parse error when an input buffer starts with a node which is not a block mapping, like the following valid YAML snippets:
(block sequence)

- 123
- foo
- 3.14

(flow sequence)

[123, foo, 3.14]

(flow mapping)

{foo: 123, true: 3.14}

So, this PR has fixed the error and added some test cases to validate the changes.

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• [ ] 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

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[github-actions[bot]]

:octocat: Upload Coverage Event Notification

Coverage data has been uploaded for the commit 7035aafeeb8ec4359f0b3800257f761af521f43a. You can download the artifact which contains the same file uploaded to the Coveralls and its HTML version.

Name	fkYAML_coverage.pr322.zip
ID	1450948658
URL	https://github.com/fktn-k/fkYAML/actions/runs/8849043964/artifacts/1450948658

[coveralls]

Pull Request Test Coverage Report for Build 8849043964

Details

• 22 of 22 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 8835266680:	0.0%
Covered Lines:	3544
Relevant Lines:	3544

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