fix k8s context checking

[bobsoppe]

Fixes the check if any k8s context has been set

Bug was introduced in commit 12f9e99 from pull request #330 where the check was changed to support fish <= 3.3.1.

[edouard-lopez]

Could you add a test in https://github.com/pure-fish/pure/blob/master/tests/_pure_prompt_k8s.test.fish to describe the behavior you expect?

before_each
@test "_pure_prompt_k8s: print nothing when context is missing" (
    set --universal pure_enable_k8s true
    set --universal pure_symbol_k8s_prefix '☸'

    # TODO
    # mock `kubectl` response
    # see https://github.com/pure-fish/pure/blob/master/tests/mocks/README.md#mocking

    _pure_prompt_k8s
) = $EMPTY

You can also check https://github.com/pure-fish/pure/blob/master/tests/_pure_k8s_namespace.test.fish and https://github.com/pure-fish/pure/blob/master/tests/_pure_k8s_context.test.fish

[edouard-lopez]

Hello, Could you resolve conflicts and add the tests?

[edouard-lopez]

@bobsoppe I don't have permission on your repo to do the rebase myself, could you do it so we can merge?

[edouard-lopez]

@bobsoppe As I can't rebase your branch, I will duplicate your fork

[edouard-lopez]

closing in favour of #334