Fix issue: 2257 Next arg flag name, no default cmd

If the next argument matches a flag name, and there is no default
command, a spurious empty string gets inserted before the flag name.

Existing code that expects the flag name in element 0 fails because it
is now in element 1 with an empty string in element 0.

When there is no default command and an empty string is detected in
element 0 (i.e. `cmd.parsedArgs.First()`), change `cmd.parsedArgs` to
omit the first empty element.

Author: bbadour

command_run.go

@@ -274,6 +274,9 @@ func (cmd *Command) run(ctx context.Context, osArgs []string) (_ context.Context
 				tracef("using default command args=%[1]q (cmd=%[2]q)", argsWithDefault, cmd.Name)
 				subCmd = cmd.Command(argsWithDefault.First())
 				cmd.parsedArgs = argsWithDefault
+				if !hasDefault && cmd.parsedArgs.Len() > 0 && cmd.parsedArgs.First() == "" {
+					&stringSliceArgs{cmd.parsedArgs.Tail()}
+				}
 			}
 		}
 	} else if cmd.DefaultCommand != "" {