client/object: Prohibit empty filter attributes in SearchV2

They are invalid according to the protocol, so no sense to pass them.

Signed-off-by: Leonard Lyubich <leonard@morphbits.io>

client/object_search.go

@@ -246,6 +246,8 @@ func (c *Client) SearchObjects(ctx context.Context, cnr cid.ID, filters object.S
 
 func verifySearchFilter(f object.SearchFilter) error {
 	switch attr := f.Header(); attr {
+	case "":
+		return errors.New("missing attribute")
 	case object.FilterContainerID, object.FilterID:
 		return fmt.Errorf("prohibited attribute %s", attr)
 	}

client/object_search_test.go

@@ -1089,6 +1089,13 @@ func TestClient_SearchObjects(t *testing.T) {
 				_, _, err = okConn.SearchObjects(ctx, anyCID, fs, []string{"attr"}, anyRequestCursor, anyValidSigner, anyValidOpts)
 				require.EqualError(t, err, "invalid filter #1: prohibited attribute $Object:objectID")
 			})
+			t.Run("empty", func(t *testing.T) {
+				var fs object.SearchFilters
+				fs.AddFilter("attr", "val", object.MatchStringEqual)
+				fs.AddFilter("", "val", object.SearchMatchType(rand.Int31()))
+				_, _, err := okConn.SearchObjects(ctx, anyCID, fs, []string{"attr"}, anyRequestCursor, anyValidSigner, anyValidOpts)
+				require.EqualError(t, err, "invalid filter #1: missing attribute")
+			})
 		})
 		t.Run("attributes", func(t *testing.T) {
 			for _, tc := range []struct {