Equip history_item_t with a default constructor

src/history.cpp

@@ -510,7 +510,7 @@ history_item_t history_impl_t::item_at_index(size_t idx) {
     }
 
     // Index past the valid range, so return an empty history item.
-    return history_item_t(wcstring(), 0);
+    return history_item_t{};
 }
 
 std::unordered_map<long, wcstring> history_impl_t::items_at_indexes(const std::vector<long> &idxs) {

src/history.h

@@ -82,7 +82,8 @@ class history_item_t {
     path_list_t required_paths;
 
    public:
-    explicit history_item_t(const wcstring &str, time_t when = 0, history_identifier_t ident = 0);
+    explicit history_item_t(const wcstring &str = wcstring(), time_t when = 0,
+                            history_identifier_t ident = 0);
 
     const wcstring &str() const { return contents; }
 

src/history_file.cpp

@@ -157,7 +157,7 @@ history_item_t history_file_contents_t::decode_item(size_t offset) const {
         case history_type_fish_1_x:
             return decode_item_fish_1_x(base, len);
     }
-    return history_item_t(L"");
+    return history_item_t{};
 }
 
 maybe_t<size_t> history_file_contents_t::offset_of_next_item(size_t *cursor, time_t cutoff) {