[Yaml] fixed handling an empty value
[Yaml] improved boolean naming ($notEOF -> !$EOF)
merged branch pvolok/fix-7274 (PR #7641)

This PR was merged into the 2.1 branch.

Discussion
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[2.2][Yaml] Fixed resolving blank values

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7274
| License       | MIT
| Doc PR        | -

Also, Seldaek suggested to rename the $notEOF variable.

Commits
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fb686d8 [Yaml] improved boolean naming ($notEOF -> !$EOF)
047212a [Yaml] fixed handling an empty value