Ensure <eztemplate> elements contains <ezcontent> child

[vidarl]

Question	Answer
JIRA issue	EZP-31130 : {{ content }} variable not always available in twig template for custom tag
Type	Improvement
Target version	1.8
BC breaks	no
Doc needed	no

This change is to ensure conversion script creates customtags in the same way as admin-ui ( even though the previous behaviour also was also valid schema wise). admin-ui always creates a <ezcontent/> element below eztemplate, even though the custom tags has no content..

eZ Platform version 2.5.6 and earlier will give fatal error on the following twig code for <eztemplate> elements which has no <ezcontent/> sub element:

{{ content|raw }}

See ezsystems/ezplatform-richtext#84 for more info.

TODO:

• Implement feature / fix a bug.
• Implement tests + specs and passing ($ composer test)
• Fix new code according to Coding Standards ($ composer fix-cs).
• Ask for Code Review.

[andrerom]

@vidarl Feel free to merge and merge up.

[lserwatka]

@vidarl could you merge it up.

[vidarl]

done