Fix non-object value of spread attributes variable

[cvlab]

Fixes first part of #2282

Why:
({ ...b }) is equivalent to (_extends({}, b))

In svelte
({ ...b }) is equivalent to (_extends(b))

Bug REPL:
https://svelte.dev/repl/b76b676843774932b0c764293ff00c48?version=3.6.9

<script>
	import Component from './Component.svelte'
	let props = { foo: 'lol' }
	
	setTimeout(() => {
		props = undefined
	})
</script>

<Component { ...props } />

Component.svelte

{$$props.foo}

$$props.foo is 'lol' even if props is already undefined

In case of static 'ObjectExpression'

<Component { ...{ foo: 'lol' } } />

{ foo: 'lol' } will not be checked at runtime if it's an object or not.

[cvlab]

• Modified mr to fix possible side effects
• Combined tests

[Rich-Harris]

Thank you! Apologies for the long wait