bug #20184 [FrameworkBundle] Convert null prefix to an empty string in translation:update (chalasr)

This PR was merged into the 2.7 branch.

Discussion
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[FrameworkBundle] Convert null prefix to an empty string in translation:update

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #20044
| License       | MIT
| Doc PR        | n/a

This command needs the ability to use an empty string as prefix, which is not possible using `bin/console translation:update --prefix=""` because `$argv` doesn't parse empty strings thus the value is converted to `null` by `ArgvInput` (only since #19946, before the option was not considered to be set, giving the default `'__'` thus this should be fine from a BC pov).

Here I propose to explicitly convert the `prefix` value to an empty string if set to `null`, as it is a very specific need and we can't guess that from `ArgvInput`.
An other way to fix it could be to add a `--no-prefix` option to the command but I don't think it is worth it, and it couldn't be treated as a bug fix thus not fixed before `3.2`.

Commits
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f02b687 [FrameworkBundle] Convert null prefix to an empty string in translation:update command

Author: fabpot

Command/TranslationUpdateCommand.php

@@ -118,8 +118,9 @@ protected function execute(InputInterface $input, OutputInterface $output)
         // load any messages from templates
         $extractedCatalogue = new MessageCatalogue($input->getArgument('locale'));
         $output->text('Parsing templates');
+        $prefix = $input->getOption('prefix');
         $extractor = $this->getContainer()->get('translation.extractor');
-        $extractor->setPrefix($input->getOption('prefix'));
+        $extractor->setPrefix(null === $prefix ? '' : $prefix);
         foreach ($transPaths as $path) {
             $path .= 'views';
             if (is_dir($path)) {