minor #36689 [Yaml] properly handle empty lines (xabbuh)

This PR was merged into the 5.1-dev branch.

Discussion
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[Yaml] properly handle empty lines

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       |
| License       | MIT
| Doc PR        |

Commits
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e91bb61 properly handle empty lines

src/Symfony/Component/Yaml/Parser.php

@@ -969,7 +969,7 @@ private function isCurrentLineBlank(): bool
     private function isCurrentLineComment(): bool
     {
         //checking explicitly the first char of the trim is faster than loops or strpos
-        $ltrimmedLine = ' ' === $this->currentLine[0] ? ltrim($this->currentLine, ' ') : $this->currentLine;
+        $ltrimmedLine = '' !== $this->currentLine && ' ' === $this->currentLine[0] ? ltrim($this->currentLine, ' ') : $this->currentLine;
 
         return '' !== $ltrimmedLine && '#' === $ltrimmedLine[0];
     }