Bugfix: throw exception on null value or cast it into string

[AlGoore]

Due to new behaviour of PHP 8.1 it is no longer allowed to pass null values into string functions like htmlspecialchars().
So this causes errors while rendering TextTemplate with a message like: htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated.
But this type of message is not really useful to find the root of the cause...

This PR changes the method TextTemplate::_parseValueOfTags().
Before a value is passed to the filters it will be checked and a TemplateParsingException is thrown or the value is just set to an empty string otherwise.
It depends on the new constructor's boolean parameter $castNullIntoString which action will be taken.

I'm not sure about the default behaviour...
I personally prefer throwing an exception 'cause there might be something wrong.
Others may prefer the casting variant / overwriting null with an empty string.
@dermatthes please take a look at the changes before merging.

[dermatthes]

Isn't it easier to just cast $input variables to string in buildinFilters.php?

TextTemplate::$__DEFAULT_FILTER["_DEFAULT_"] = function ($input) { return htmlspecialchars($input); };

// Raw is only a pseudo-filter. If it is not in the chain of filters, __DEFAULT__ will be appended to the filter
TextTemplate::$__DEFAULT_FILTER["html"] = function ($input) { return htmlspecialchars($input); };
TextTemplate::$__DEFAULT_FILTER["raw"] = function ($input) { return $input; };
TextTemplate::$__DEFAULT_FILTER["singleLine"] = function ($input) { return str_replace("\n", " ", $input); };
TextTemplate::$__DEFAULT_FILTER["inivalue"] = function ($input) { return addslashes(str_replace("\n", " ", $input)); };

To

TextTemplate::$__DEFAULT_FILTER["_DEFAULT_"] = function ($input) { return htmlspecialchars( (string) $input); };
...

Or do you see a special benefit from introducing a dedicated configuration parameter?