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%.
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Please don't

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[github-actions]

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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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