Removing a component's last style leaves the old CSS applied until a full reload

[Brooooooklyn]

Split out of a Codex review finding on #455. Pre-existing on main — the transition behaves identically there.

Symptom

A component has styles, then loses them — its last inline styles entry is deleted, its array becomes [], or its external stylesheet becomes empty. HMR updates the component, but the old CSS stays applied. Only a full reload clears it.

Cause

The HMR module spreads the existing Class.ɵcmp, so a property it does not emit keeps its previous value. The generator omits styles for BOTH null and an empty array:

compileForHmrSync(styles = null)        →  no styles property emitted
compileForHmrSync(styles = [])          →  no styles property emitted
compileForHmrSync(styles = ['.a{}'])    →  styles: [ … ]

So there is no way for the plugin to express "this component now has no styles" — the distinction is erased on the Rust side.

Why the obvious fix is not enough

The review that surfaced this recommended preserving a definitive empty state through compileForHmrSync and emitting styles: []. The first half is a plugin change; the second half is a Rust generator change, and without it the plugin cannot express the state no matter what it passes. Both halves are needed.

Suggested fix

1. Make the HMR generator emit styles: [] when it is given an empty array, so the spread is overridden, while continuing to omit the property for null (meaning "unknown, leave as-is").
2. Have the plugin pass [] rather than null when it knows the component is definitively styleless — fix(vite): resolve @ng/component styles per class #455 already computes that state and currently collapses it to null.

Worth adding endpoint regressions for both transitions, non-empty to empty external and non-empty to empty inline. #455's tests only cover components that were already styleless, so they cannot catch this.