[YAML] Improve performance of getNextEmbedBlock

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

By removing unnecessary preg_match and function calls - isCurrentLineEmpty() contains a call to isCurrentLineBlank() - therefore this function is called twice every time this condition is hit. The preg_match appears to legacy handling of blank lines.

This improves the performance of the Drupal 8 installer.
![image](https://f.cloud.github.com/assets/769634/2241426/69effb0c-9cd1-11e3-9145-e4fabd2ec870.png)