fix: sidebar 'auto' not working (#178)

[kiaking]

fix #178

Sidebar gets hidden because auto was treated as empty. Added helper function to check if the sidebar is empty or not, and refactored a bit on checking logic for better readability 👀

This adds a breaking change where undefined sidebar now resolves to auto where currently it's treated as false. Though VuePress treats undefined sidebar as auto so this is how it should have been 😓

[cloydlau]

Has this been removed later on? I got a TypeError: Cannot read properties of undefined (reading 'some') with the following config:

sidebar: {
  'XXX': 'auto',
}