bip158: Return no match for empty query

Perhaps this is up for debate, but considering the use case of checking
for inclusions in a block, an empty query should not return `true` for any
element, as no elements exist in the query. If the user happens to have
no scripts to query for whatever reason, they will think every block is
relevant.

Author: rustaceanrob

bitcoin/src/bip158.rs

@@ -306,7 +306,7 @@ impl GcsFilterReader {
         // sort
         mapped.sort_unstable();
         if mapped.is_empty() {
-            return Ok(true);
+            return Ok(false);
         }
         if n_elements == 0 {
             return Ok(false);