minor #18291 Fix copy-paste in PHPDoc comment text (guilliamxavier)

This PR was merged into the 2.3 branch.

Discussion
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Fix copy-paste in PHPDoc comment text

| Q             | A
| ------------- | ---
| Branch?       | 2.3+ (actually all branches from 2.0)
| License       | MIT

In http://api.symfony.com/2.3/Symfony/Component/Form/DataTransformerInterface.html, compare the documentation (*emphasis* mine) respectively for `transform` and for `reverseTransform`:

> Usually this will be *NULL*, but […] other empty values are possible as well (such as *empty strings*).
> Usually this will be *an empty string*, but […] other empty values are possible as well (such as *empty strings*).

This PR corrects the repetition in the latter, giving symmetry:

> Usually this will be *NULL*, but […] other empty values are possible as well (such as *empty strings*).
> Usually this will be *an empty string*, but […] other empty values are possible as well (such as *NULL*).

_Note:_ For now I just changed `empty strings` to `NULL`, which is 9 characters shorter, should I also reformat the affected comment paragraph so that this line and the subsequent ones fill up to 80 columns (as was the case before the change, and is for `transform`)?

Commits
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3f6e37e Fix copy-paste in PHPDoc comment text

Author: Tobion

src/Symfony/Component/Form/DataTransformerInterface.php

@@ -58,7 +58,7 @@ public function transform($value);
      *
      * This method must be able to deal with empty values. Usually this will
      * be an empty string, but depending on your implementation other empty
-     * values are possible as well (such as empty strings). The reasoning behind
+     * values are possible as well (such as NULL). The reasoning behind
      * this is that value transformers must be chainable. If the
      * reverseTransform() method of the first value transformer outputs an
      * empty string, the second value transformer must be able to process that