Do not allow empty items to be added to history

Empty items are used as sentinels to indicate that we've reached the end of
history, so they should not be added as actual items. Enforce this.

Fixes #6032

src/fish_tests.cpp

@@ -3654,6 +3654,10 @@ void history_tests_t::test_history() {
     }
     history.save();
 
+    // Empty items should just be dropped (#6032).
+    history.add(L"");
+    do_test(!history.item_at_index(1).contents.empty());
+
     // Read items back in reverse order and ensure they're the same.
     for (i = 100; i >= 1; i--) {
         history_item_t item = history.item_at_index(i);