[v2] Alpha 66 breaks classic site without logo

[SamChou19815]

🐛 Bug Report

In #3394, we no longer give logo field a dummy empty object if it's undefined, which will cause it to crash at line 19:

docusaurus/packages/docusaurus-theme-classic/src/theme/hooks/useLogo.ts

Lines 15 to 19 in 1444770

	const {
	navbar: {logo},
	} = useThemeConfig();
	const {isDarkTheme} = useThemeContext();
	const logoLink = useBaseUrl(logo.href || '/');

Have you read the Contributing Guidelines on issues?

Yes

To Reproduce

Upgrade a classic site without logo to alpha-66.

Expected behavior

Sites without logo property should continue to work

Actual Behavior

Blank screen with js error:

Your Environment

• Docusaurus version used: v2-alpha-66
• Environment name and version (e.g. Chrome 78.0.3904.108, Node.js 10.17.0): Chrome
• Operating system and version (desktop or mobile): macOS

Reproducible Demo

https://github.com/SamChou19815/website/tree/master/packages/blog

Bump it to alpha-66, and yarn start.