[BUGFIX] make ContentArgumentName non required argument

[Kanti]

if you have a ViewHelper that has a required argument like 'record'.
And you use

    public function getContentArgumentName(): string
    {
        return 'record';
    }

and then you want to use:

{record -> my:vh()}

you currently get this error:

Fluid parse error in template template_..., line 19 at character 1. Error: Required argument "record" was not supplied. (error code 1237823699). Template source chunk: ...

[s2b]

As discussed: I'd rather not do it like this and instead fix it properly with Fluid 6, see #1303.