fix(river/tags): parse hide-vacant once, not in the wl callbacks

config_["hide-vacant"].asBool() was called from the river status listeners.
When the option is given as a string ("true") jsoncpp's asBool() throws
"Value is not convertible to bool", and that exception unwinding through
libwayland's C dispatch aborts the process. Parse it once in the constructor
into a bool member (accepting the string form) and read the cached value.

Fixes #4078

include/modules/river/tags.hpp

@@ -38,6 +38,7 @@ class Tags : public waybar::AModule {
   std::vector<Gtk::Button> buttons_;
   struct zriver_output_status_v1* output_status_;
   struct zriver_seat_status_v1* seat_status_;
+  bool hide_vacant_ = false;  // parsed once; asBool() in a wl callback would throw (#4078)
 };
 
 } /* namespace waybar::modules::river */