fix($core): cannot read property 'path' of undefined

[Raiondesu]

Summary

When layout is not present in a theme, the following shows up in console:

tip Apply theme vuepress-theme-.vuepress (extends vuepress-theme-vuepress-theme) ...
TypeError: Cannot read property 'path' of undefined
    at ThemeAPI.getLayoutComponentMap (@vuepress\core\lib\node\theme-api\index.js:83:67)
    at ThemeAPI.init (@vuepress\core\lib\node\theme-api\index.js:35:36)
    at new ThemeAPI (@vuepress\core\lib\node\theme-api\index.js:17:10)
    at loadTheme (@vuepress\core\lib\node\loadTheme.js:59:10)
    at App.process (@vuepress\core\lib\node\App.js:100:21)
    at dev (@vuepress\core\lib\index.js:14:13)
    at args (vuepress\lib\util.js:35:12)
    at CAC.cli.command.option.option.option.option.option.option.option.option.option.action (vuepress\lib\registerCoreCommands.js:38:23)
    at CAC.runMatchedCommand (cac\dist\index.js:700:38)
    at CAC.parse (cac\dist\index.js:620:18)
    at CLI (vuepress\lib\util.js:23:7)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:189:7)

This PR fixes this issue, by removing a reference to an obvious undefined value (Layout).

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:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

This issue does not correspond to a browser, it's a node-related problem

• Chrome
• Firefox
• Safari
• Edge
• IE

[vue-bot]

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