NcActions with tertiary-no-background has background when open

[susnux]

When using the type tertiary-no-background I would expect the actions toggle to never have a background assigned, but when the actions are toggled the button gets the default background assigned.

This looks a bit odd in my case:

[raimund-schluessler]

That's a question for @nextcloud/designers. But I have to agree. It would be more consistent, as the NcButton component of type tertiary-no-background has no background on click either.

The fix would be to add

	&.action-item--tertiary-no-background {
		--open-background-color: transparent;
	}

before this:
https://github.com/nextcloud/nextcloud-vue/blob/27d09d66f582cedccdcb34223368d70daab06904/src/components/NcActions/NcActions.vue#L1172-L1175

[raimund-schluessler]

I took the liberty to create a PR, so one can check it out live: #3724.