Fix baseUri scheme related error

[adriendupuis]

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Since #1931 and PHP 8, an error occurs when building HTML:

PHP Deprecated:  mb_strtoupper(): Passing null to parameter #1 ($string) of type string is deprecated in tools/raml2html/vendor/raml-org/raml-php-parser/src/ApiDefinition.php on line 813

• baseUri is supposed to be the online API entry point absolute URL. This will never be our case.
• baseUri is mandatory.
• baseUri is actually used to set the Ibexa logo link. I didn't manage to pass this link into another attribute. And, as it's mandatory, let's keep baseUri for this link.

The patch avoid having the error on a relative baseUri like ours.

Checklist

• Text renders correctly
• Text has been checked with vale
• Description metadata is up to date
• Redirects cover removed/moved pages
• Code samples are working
• PHP code samples have been fixed with PHP CS fixer
• Added link to this PR in relevant JIRA ticket or code PR