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`.
switch before_script to before_install and script to install
minor #11826 switch before_script to before_install and script to install (cordoval)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #11826).

Discussion
----------

switch before_script to before_install and script to install

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

Commits
-------

e2ee13a switch before_script to before_install and script to install
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
[Console] guarded against invalid aliases
bug #11874 [Console] guarded against non-traversable aliases (thierrymarianne)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #11874).

Discussion
----------

[Console] guarded against non-traversable aliases

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

This patch would guard against passing non-traversable aliases to `setAliases` method.
It follows discussion about a previous [PR](https://github.com/symfony/symfony/pull/11873) which broke BC.

Commits
-------

9081014 [Console] guarded against invalid aliases
[Intl] Improved bundle reader implementations
bug #11907 [Intl] Improved bundle reader implementations (webmozart)

This PR was merged into the 2.3 branch.

Discussion
----------

[Intl] Improved bundle reader implementations

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

This PR extracts bundle reader improvements from #9206.

The code is internal and used for resource bundle generation only, so I did not care about BC too much.

Commits
-------

c3cce5c [Intl] Improved bundle reader implementations
[Intl] Integrated ICU data into Intl component