The default value for array options will be an array, so it is not suitable to use as the default when processing one of many values for a multi-value option. Using null seems appropriate here, as it indicates the absence of a value and also converts nicely to an empty string (as opposed to an empty array).
Fixes#7689
* 2.0:
[Yaml] fixed default value
Added Yaml\Dumper::setIndentation() method to allow a custom indentation level of nested nodes.
added a way to enable/disable object support when parsing/dumping
added a way to enable/disable PHP support when parsing a YAML input via Yaml::parse()
[Console] fixed input bug when the value of an option is empty (closes#6649, closes#6689)
Conflicts:
src/Symfony/Component/Yaml/Dumper.php
src/Symfony/Component/Yaml/Parser.php
src/Symfony/Component/Yaml/Tests/DumperTest.php
src/Symfony/Component/Yaml/Tests/ParserTest.php
src/Symfony/Component/Yaml/Yaml.php