Fix command description

- Align default value between declarative and programmatic approaches
- Remove unnecessary help display in command completion

Resolves #1236

Author: fmbenhassine

spring-shell-core/src/main/java/org/springframework/shell/core/command/Command.java

@@ -52,7 +52,7 @@ default String getName() {
 	 * @return the description of the command
 	 */
 	default String getDescription() {
-		return "";
+		return "N/A";
 	}
 
 	/**
@@ -137,7 +137,7 @@ final class Builder {
 
 		private String name = "";
 
-		private String description = "";
+		private String description = "N/A";
 
 		private String group = "";
 

spring-shell-jline/src/main/java/org/springframework/shell/jline/CommandCompleter.java

@@ -65,7 +65,7 @@ public void complete(LineReader reader, ParsedLine line, List<Candidate> candida
 		else {
 			for (Command command : this.commandRegistry.getCommands()) {
 				candidates.add(new Candidate(command.getName(), command.getName() + ": " + command.getDescription(),
-						command.getGroup(), command.getHelp(), null, null, true));
+						command.getGroup(), null, null, null, true));
 			}
 		}
 	}