image/tree: Fix width calculation for untagged images

When calculating column widths for the tree view, untagged images
weren't being properly accounted for in the width calculation.

This caused layout issues when there were tagged images were shorter
than the `<untagged>` string.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

cli/command/image/tree.go

@@ -547,7 +547,11 @@ func (h imgColumn) PrintR(clr aec.ANSI, s string) string {
 func widestFirstColumnValue(headers []imgColumn, images []topImage) int {
 	width := len(headers[0].Title)
 	for _, img := range images {
-		for _, name := range img.Names {
+		names := img.Names
+		if len(names) == 0 {
+			names = []string{untaggedName}
+		}
+		for _, name := range names {
 			if len(name) > width {
 				width = len(name)
 			}