[Fix] Ensure string resolution of Lazy_String

[amiut]

Lazy_String::__toString() does not resolve to string in some cases which causes fatal error where string is expected. In our case problem occurs in community add event page where Event object's properties like title, permalink, excerpt, etc. are empty and the problem happens on wp_set_cache level. For example look at this line, the result of tribe_events_get_the_excerpt function is possible to be null which makes Lazy_String::__toString() return null as well.