Make params argument of trigger_client_event() optional

[cjtapper]

This PR adds support for triggering a client event with no params without having to call the trigger_client_event with a params argument. It also adds support for using string rather than JSON value for the HX-Trigger* when there is only a single event with no params (see https://htmx.org/headers/hx-trigger/).

# Before this PR
>>> response = HttpResponse()
>>> response = trigger_client_event(response, "showConfetti", {})
>>> response.headers["HX-Trigger"]
'{"showConfetti": {}}'

# After this PR
>>> response = HttpResponse()
>>> response = trigger_client_event(response, "showConfetti")
>>> response.headers["HX-Trigger"]
"showConfetti"

# Also handles promoting of single events to JSON if additional events are added
>>> response = HttpResponse()
>>> response = trigger_client_event(response, "showConfetti")
>>> response.headers["HX-Trigger"]
"showConfetti"
>>> response = trigger_client_event(response, "showMessage", {"value": "Hello!"})
>>> response.headers["HX-Trigger"]
'{"showConfetti": {}, "showMessage": {"value": "Hello!"}}'

I chose to set the default value of promoted events to {} rather than None (or something else), as this mirrors how HTMX itself handles events with no params (it sets the details to an empty object; see https://github.com/bigskysoftware/htmx/blob/60acaacc416812e85245f8e00d1e8bc4954d1d83/src/htmx.js#L992-L1008)

I realise that the docs will also need to be updated - I'm happy to do this as well, but just wanted to check that this would actually be accepted first :)

[lolrenx]

Good stuff, the empty trailing payload has been triggering me for a while

[adamchainz]

It's quite complicated to try supporting the non-JSON form of the header. It looks like we could just send HX-Trigger: {"event": null} or {"event": {}} if params aren't provided, which is a much smaller change?

[cjtapper]

Would certainly simplify things. Supporting the non-JSON form is probably an unnecessary micro-optimisation, though one advantage of it is that it's slightly less cumbersome to assert for in tests.

We'll want to use {"event": {}} rather than {"event": null} if we want HTMX to behave exactly the same way as it would for the non-JSON version.

[adamchainz]

Would certainly simplify things. Supporting the non-JSON form is probably an unnecessary micro-optimisation, though one advantage of it is that it's slightly less cumbersome to assert for in tests.

Tests will just assert for the raw string {"event: {}}, no?

We'll want to use {"event": {}} rather than {"event": null} if we want HTMX to behave exactly the same way as it would for the non-JSON version.

Okay, right. Can you do it then? Please also rebase, and add changelog note and docs 😇

[cjtapper]

Yep, will have some time to make these changes a bit later today

[cjtapper]

Well, this has ended up being a much smaller diff. I guess it saves me from typing four extra characters every time I call trigger_client_event with no params 😅.

There wasn't an example in the changelog for how you like to list consecutive PRs from the one contributor, so I just included the same "thanks to..." message a second time.

[adamchainz]

Thank you very much!

There wasn't an example in the changelog for how you like to list consecutive PRs from the one contributor, so I just included the same "thanks to..." message a second time.

Yup this is good.