Fix Wildcard Queries Against The Default Field (elastic#24778)

When querying against the default field (i.e. querying without specifying a field) we weren't correctly handling values with wildcards in them.

src/ui/public/kuery/functions/__tests__/is.js

@@ -96,6 +96,18 @@ describe('kuery functions', function () {
         expectDeepEqual(result, expected);
       });
 
+      it('should return an ES query_string query using default_field when fieldName is null and value contains a wildcard', function () {
+        const expected = {
+          query_string: {
+            query: 'jpg*',
+          }
+        };
+
+        const node = nodeTypes.function.buildNode('is', null, 'jpg*');
+        const result = is.toElasticsearchQuery(node, indexPattern);
+        expectDeepEqual(result, expected);
+      });
+
       it('should return an ES bool query with a sub-query for each field when fieldName is "*"', function () {
         const node = nodeTypes.function.buildNode('is', '*', 200);
         const result = is.toElasticsearchQuery(node, indexPattern);

src/ui/public/kuery/functions/is.js

@@ -48,6 +48,14 @@ export function toElasticsearchQuery(node, indexPattern) {
   const type = isPhraseArg.value ? 'phrase' : 'best_fields';
 
   if (fieldNameArg.value === null) {
+    if (valueArg.type === 'wildcard') {
+      return {
+        query_string: {
+          query: wildcard.toQueryStringQuery(valueArg),
+        },
+      };
+    }
+
     return {
       multi_match: {
         type,