Don't try to match with an empty query string

This was causing UserProvider to give results because every string happens to start with empty string and its regex also acepts the empty string.

src/autocomplete/QueryMatcher.js

@@ -86,6 +86,9 @@ export default class QueryMatcher {
         if (this.options.shouldMatchWordsOnly) {
             query = query.replace(/[^\w]/g, '');
         }
+        if (query.length === 0) {
+            return [];
+        }
         const results = [];
         this.keyMap.keys.forEach((key) => {
             let resultKey = key.toLowerCase();