Fixed no-side-effects-in-computed-properties when el is object

[ktmouk]

Hi, thanks for great plugin!

I'm using object property to set el of Vue.
However, I get a TypeError when I update eslint-plugin-vue to v7.0.0-alpha.7 and run eslint.

for example:

const dummy = { app: '#app' }

new Vue({
  el: dummy.app,
  template: '<p>test</p>',
});

TypeError: Cannot read property 'body' of undefined

I think the cause of this issue is that the scopeStack has no default value.
but, I’m not sure if this PR is the best solution.

Could you please review this PR or use another best solution to fix this issue?
Thanks.

[ktmouk]

I referred to

eslint-plugin-vue/lib/rules/require-valid-default-prop.js

Line 109 in eb20820

/** @type { { upper: any, body: null | BlockStatement | Expression, returnTypes?: null | ReturnType[] } } */

[ota-meshi]

Thank you!