EZP-31130: Ensured content variable is always present in Custom Template Twig view

[alongosz]

Question	Answer
JIRA issue	EZP-31130
Bug/Improvement	yes
New feature	no
Target version	v1.1.x for eZ Platform 2.5.x LTS
BC breaks	no
Tests pass	yes
Doc needed	no, doc is already aligned

This PR introduces small change - ensuring that content Twig variable is always present in Custom Template (Tag, Style) Twig view.

eZ Platform DocBook schema (.rng) currently allows ezcontent tag to be not present in eztemplate. While AdminUI ensures ezcontent is always set, even if empty, a DocBook comming from migrated XmlText might not have this tag sometimes. This resulted in content Twig variable being not present in a Twig view for a given Custom Template, which yields a fatal error both on a front site and in AdminUI Location View.

There were 3 directions to solve that issue:

1. Force a developer to check for existence of that variable.
2. Add ezcontent tag during migration from XmlText to RichText.
3. Make content variable always present in a Twig view for Custom Template.

The second option was rejected because RNG schema defines ezcontent as optional.
The first option yields bad DX.

Currently this PR implements the third option, so a developer, instead of writing:

{% if content is defined %}{{ content|raw }}{% endif %}

can simply write:

{{ content|raw }}

which is also consistent with the doc that we provide.

QA

• Sanity checks for creating Custom Tags with or without content.
• Test: Create a Custom Tag, find it in ezcontentobject_attributes, remove <content>...</content> markup from XML, save, clear cache, observe the mentioned error on a front site.

TODO:

• Always expose content variable in Twig views for Custom Templates.
• Align tests.
• Rebase after merging EZP-31108: [CS] Enabled ezsystems/ezplatform-code-style package #83
• Fix new code according to Coding Standards ($ composer fix-cs).
• Ask for Code Review.

[vidarl]

Looks fine and works here ;)

[vidarl]

FYI : I just realized that AdminUI does NOT ensure <ezcontent/>is set on inline custom tags if they have no content

[alongosz]

FYI : I just realized that AdminUI does NOT ensure <ezcontent/>is set on inline custom tags if they have no content

Hmm, ok. Just one more reason for the change proposed here 👍

[lserwatka]

You can merge it up.

[alongosz]

You can merge it up.

Done via 0b1045f.