Within the Algolia Dashboard you'll find an Analytics tab dedicated to searches with no results.
Sometimes, the addition of a filter to a search causes the search to start returning no results. In cases like these, we add a note to the no results section to indicate how many of the no results searches had a filter applied if the number of filtered searches matches the total then this suggests that it is only searches with an additional filter added that are not returning results: 
You can also click on a search to gain more information on any filters that were used with a search that returned no results
You can also use the Analytics REST API get top searches with no results endpoint.
We have a guide here for reducing no results searches that you might find useful.