MRG: fix calculate_gather_stats threshold=0 bug (#3052)

The error shows up when we try to calculate abundance info with a
`match_size` of 0:
```
match: GCF_002819265.1 Choclo virus, match_size: 0, query_size: 273, query_trackabund: true
thread '<unnamed>' panicked at 'called `Option::unwrap()` on a `None` value', /home/ntpierce/sourmash/src/core/src/index/mod.rs:301:55
```

...we should never be trying to calculate statistics on `match_size` of
0, but the logic is currently `match_size >= threshold`.This includes a
check to break if `match_size` is 0.

- fixes #3051

src/core/src/index/mod.rs

@@ -280,13 +280,14 @@ pub fn calculate_gather_stats(
 
     // If abundance, calculate abund-related metrics (vs current query)
     if calc_abund_stats {
-        // need current downsampled query here to get f_unique_weighted
+        // take abunds from subtracted query
         let (abunds, unique_weighted_found) = match match_mh.inflated_abundances(&query) {
             Ok((abunds, unique_weighted_found)) => (abunds, unique_weighted_found),
             Err(e) => {
                 return Err(e);
             }
         };
+
         n_unique_weighted_found = unique_weighted_found as usize;
         sum_total_weighted_found = sum_weighted_found + n_unique_weighted_found;
         f_unique_weighted = n_unique_weighted_found as f64 / total_weighted_hashes as f64;

src/core/src/index/revindex/disk_revindex.rs

@@ -330,6 +330,10 @@ impl RevIndexOps for RevIndex {
 
             let (dataset_id, size) = counter.k_most_common_ordered(1)[0];
             match_size = if size >= threshold { size } else { break };
+            // handle special case where threshold was set to 0
+            if match_size == 0 {
+                break;
+            }
 
             // this should downsample mh for us
             let match_sig = self.collection.sig_for_dataset(dataset_id)?;