Subcommands are not shown in usage unless they have their help text specified

[arocnies]

It seems commands are only listed in the usage if they have non-empty help text.
Steps to reproduce:

1. Copy tool example from https://ajalt.github.io/clikt/commands/
2. Run the main method and see that the usage does not contain the execute command
3. Add non-empty help text to Execute class (e.g. CliktCommand(help = "Foo")
4. Run the main method and see the usage now contains the execute command

Expected behavior (using the "tool" example from https://ajalt.github.io/clikt/commands/):

$ ./tool
Usage: tool [OPTIONS] COMMAND [ARGS]...

Options:
--verbose / --no-verbose
-h, --help Show this message and exit

Commands:
execute

Actual:

$ ./tool
Usage: tool [OPTIONS] COMMAND [ARGS]...

Options:

-h, --help Show this message and exit

Commands:

[ajalt]

Thanks for the report!