Fixed parsing of leading blank lines in folded scalars.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7989
| License       | MIT
| Doc PR        | n/a

Fixed the issue raised in #7989. The cause was in the way the parser determined the indentation level of block scalars. When the indentation is not manually specified, the parser now determines the indentation from the first *non-empty* line as per section [8.1.1.1.](http://www.yaml.org/spec/1.2/spec.html#id2793979) of the spec.