fix handling of empty sequence items

When a line contains only a dash it cannot safely be assumed that
it contains a nested list or an embedded mapping. If the next line
starts with a dash at the same indentation, the current line's item
is to be treated as `null`.
improve error when detecting unquoted asterisks

Asterisks in unquoted strings are used in YAML to reference
variables. Before Symfony 2.3.19, Symfony 2.4.9 and Symfony 2.5.4,
unquoted asterisks in inlined YAML code were treated as regular
strings. This was fixed for the inline parser in #11677. However, an
unquoted * character now led to an error message like this:

```
PHP Warning:  array_key_exists(): The first argument should be either a string or an integer in vendor/symfony/symfony/src/Symfony/Component/Yaml/Inline.php on line 409

  [Symfony\Component\Yaml\Exception\ParseException]
  Reference "" does not exist at line 171 (near "- { foo: * }").
```
properly mask escape sequences in quoted strings
bug #11799 [YAML] fix handling of empty sequence items (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

[YAML] fix handling of empty sequence items

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11798
| License       | MIT
| Doc PR        |

When a line contains only a dash it cannot safely be assumed that it contains a nested list or an embedded mapping. If the next line starts with a dash at the same indentation, the current line's item is to be treated as `null`.

Commits
-------

fc85435 fix handling of empty sequence items
Merge branch '2.3' into 2.4

* 2.3: (35 commits)
  [Form] Fix PHPDoc for builder setData methods The underlying data variable is typed as mixed whereas the methods paramers where typed as array.
  fixed CS
  [Intl] Improved bundle reader implementations
  [Console] guarded against invalid aliases
  switch before_script to before_install and script to install
  fixed typo
  [HttpFoundation] Request - URI - comment improvements
  [Security] Added more tests
  remove `service` parameter type from XSD
  [Intl] Added exception handler to command line scripts
  [Intl] Fixed a few bugs in TextBundleWriter
  [Intl] Updated icu.ini up to ICU 53
  [Intl] Removed non-working $fallback argument from ArrayAccessibleResourceBundle
  Use separated function to resolve command and related arguments
  [SwiftmailerBridge] Bump allowed versions of swiftmailer
  [FrameworkBundle] Remove invalid markup
  [Intl] Added "internal" tag to all classes under Symfony\Component\Intl\ResourceBundle
  Remove routes for removed WebProfiler actions
  [Security] Fix usage of unexistent method in DoctrineAclCache.
  backport more error information from 2.6 to 2.3
  ...

Conflicts:
	.travis.yml
	src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php
	src/Symfony/Component/DependencyInjection/Tests/Loader/XmlFileLoaderTest.php
	src/Symfony/Component/HttpKernel/Kernel.php
	src/Symfony/Component/Process/PhpExecutableFinder.php