rpk: add name by default to the prompt if not empty

If a user has a prompt but no "%p" nor "%n" then
we will print the name.

src/go/rpk/pkg/cli/profile/prompt.go

@@ -189,10 +189,12 @@ func parsePrompt(s string, name string) (string, []color.Attribute, error) {
 	}
 
 	output := make([]byte, 0, len(s)+len(text))
+	var withEscape bool
 	for i := 0; i < len(text); i++ {
 		c := text[i]
 		switch c {
 		case '%':
+			withEscape = true
 			if len(text) > i+1 {
 				switch text[i+1] {
 				case 'p', 'n':
@@ -209,6 +211,9 @@ func parsePrompt(s string, name string) (string, []color.Attribute, error) {
 			output = append(output, c)
 		}
 	}
+	if !withEscape {
+		output = append(output, name...)
+	}
 	return string(output), attrs, nil
 }