perf(template-compiler): make empty children implicit

[nolanlawson]

Details

Fixes #2671

In the template compiler, calls to api_element, api_custom_element, and api_dynamic_element no longer render an empty array ([]) when the children are empty. This removes some boilerplate from generated templates.

I don't expect this to improve runtime performance much (if at all), but the generated code is smaller, so it's definitely more performant from that perspective.

Does this pull request introduce a breaking change?

• ✅ No, it does not introduce a breaking change.

Does this pull request introduce an observable change?

• ✅ No, it does not introduce an observable change.

GUS work item

W-10660536

[nolanlawson]

FWIW, the api_custom_element API already had the EmptyArray default:

lwc/packages/@lwc/engine-core/src/framework/api.ts

Lines 165 to 171 in 19e8a73

	// [c]ustom element node
	function c(
	sel: string,
	Ctor: LightningElementConstructor,
	data: VElementData,
	children: VNodes = EmptyArray
	): VCustomElement {

... but it was always being called with the children argument.

[nolanlawson]

Perf tests, just to be sure:

(100 sample size, 15 min timeout, 1% condition)

I'm going to assume that that 1% is just variance because the test is so short (<100ms).