Fix usage() for missing description on commands

Before this commit, if you supply a command to a JCommander object
and you dont supply a description for the command, then when calling
usage() you will see a line of the following format:
"<command-name> null"

After this commit the string literal "null" will be omitted from the
usage() call if you do not supply a description for the command.

Fixes #480

src/main/java/com/beust/jcommander/DefaultUsageFormatter.java

@@ -266,7 +266,10 @@ public void appendCommands(StringBuilder out, int indentCount, int descriptionIn
             if (p == null || !p.hidden()) {
                 JCommander.ProgramName progName = commands.getKey();
                 String dispName = progName.getDisplayName();
-                String description = indent + s(4) + dispName + s(6) + getCommandDescription(progName.getName());
+                String commandDescription = Optional.ofNullable(getCommandDescription(progName.getName()))
+                    .map(desc -> s(6) + desc)
+                    .orElse("");
+                String description = indent + s(4) + dispName + commandDescription;
                 wrapDescription(out, indentCount + descriptionIndent, description);
                 out.append("\n");