context command panics when invoked without a context name

[arunvelsriram]

❯ RUST_BACKTRACE=1 click
[none] [none] [none] > context
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', libcore/option.rs:335:21
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
   1: std::sys_common::backtrace::_print
   2: std::panicking::default_hook::{{closure}}
   3: std::panicking::default_hook
   4: std::panicking::rust_panic_with_hook
   5: std::panicking::begin_panic
   6: std::panicking::begin_panic_fmt
   7: rust_begin_unwind
   8: core::panicking::panic_fmt
   9: core::panicking::panic
  10: core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &'a mut F>::call_once
  11: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  12: <click::cmd::Context as click::cmd::Cmd>::exec
  13: click::main
  14: std::rt::lang_start::{{closure}}
  15: std::panicking::try::do_call
  16: __rust_maybe_catch_panic
  17: std::rt::lang_start_internal
  18: main

The expected behaviour could be same as that of namespace command. namespace when invoked without an argument, sets the namespace to none.

[nicklan]

it actually now just prints out all the available contexts.