[BUG] Wrong interface on compileSource leads to fatal error

[oleghind]

protected function compileSource(string! viewCode, bool extendsMode = false) -> string

compileSource method returns string, but sometimes it returns array

Example:

                    if extendsMode {
                        let finalCompilation[] = block;
                    } else {
                        let finalCompilation .= block;
                    }
                }
            }

            return finalCompilation;

... which leads to a crash:
PHP Fatal error: Phalcon\Mvc\View\Engine\Volt\Compiler::compileSource(): Return value must be of type string, array returned

Better adjust interface to string | array

[niden]

Resolved in #16609

Thank you @oleghind