history: remove bogus assertion crashing on empty history items

In LastC++11, an empty history item means we either reached the end of history,
or the item is actually empty. The second meaning is still true.  We never
append empty history items but the history file might have been modified.

Fixes #10129

fish-rust/src/history.rs

@@ -2032,8 +2032,6 @@ pub fn go_to_next_match(&mut self, direction: SearchDirection) -> bool {
                 return false;
             };
 
-            assert!(!item.is_empty());
-
             // Look for an item that matches and (if deduping) that we haven't seen before.
             if !item.matches_search(&self.canon_term, self.search_type, !self.ignores_case()) {
                 continue;