minor #21754 [Serializer] Reduce nesting in YamlFileLoader (gadelat)
minor #21754 [Serializer] Reduce nesting in YamlFileLoader (gadelat)

This PR was merged into the 2.7 branch.

Discussion
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[Serializer] Reduce nesting in YamlFileLoader

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

We don't need to check if $this->classes is empty, because isset takes care of it in next call anyway

Diffs on GH are hard to read for this type of change, here is old and new code:

```php
public function loadClassMetadata(ClassMetadataInterface $classMetadata)
{
    if (null === $this->classes) {
        $this->classes = $this->getClassesFromYaml();
    }

    if (!$this->classes) {
        return false;
    }

    if (isset($this->classes[$classMetadata->getName()])) {
        $yaml = $this->classes[$classMetadata->getName()];

        if (isset($yaml['attributes']) && is_array($yaml['attributes'])) {
           ...
        }

        return true;
    }

    return false;
}
```

```php
public function loadClassMetadata(ClassMetadataInterface $classMetadata)
{
    if (null === $this->classes) {
        $this->classes = $this->getClassesFromYaml();
    }

    if (!isset($this->classes[$classMetadata->getName()])) {
        return false;
    }

    $yaml = $this->classes[$classMetadata->getName()];

    if (isset($yaml['attributes']) && is_array($yaml['attributes'])) {
        ...
    }

    return true;
}
```

Commits
-------

45f0b16 [Serializer] Reduce nesting in YamlFileLoader

symfony/Serializer,symfony/serializer