fix($plugin-pwa): no global-ui-component when updatePopup is disabled

[meteorlxy]

Summary

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

Other information:

Disable updatePopup in config.js:

['@vuepress/pwa', {
  updatePopup: false
}],

Before

index.html that generated by build:

<div class="global-ui"><!----><SWUpdatePopup></SWUpdatePopup></div>

After

index.html that generated by build:

<div class="global-ui"><!----></div>

[ulivz]

LGTM

[vue-bot]

Hey @meteorlxy, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚