Make CommandTester::execute input argument use a default value

[kobelobster]

• Transform traditional array syntax to short array syntax
• Add PHP 7.1 return types
• Make input argument of CommandTester::execute use an empty array per default
• Make code PSR-2 conform

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The reason for this PR is that I stumbled upon this documentation https://symfony.com/doc/current/console.html#testing-commands where the example shows

$commandTester->execute(array( 'command' => $command->getName(), // pass arguments to the helper 'username' => 'Wouter', // prefix the key with two dashes when passing options, // e.g: '--some-option' => 'option_value', ));

However, I saw, that the execute method already takes the name of the command passed in the constructor if no argument is passed, which means, for commands without input options you can just pass an empty array, therefore you could also make the default value of that argument an empty array.

I fixed PSR-2, return types and short array syntax as well, but don't know how Symfony handles that. I did that because composer.json requires php >=7.1 anyway, so all the features should be available, but can change it back if it's not within Symfony standards to do that.

[symfony-bot]

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