vnode.type TypeError fixed

[Mhmdrza]

typeof vnode.type will throw an Execption if vnode being null. A simple truthy check fixes the issue.

It's a bug fix. related to async-loader.

Did you add tests for your changes? No

Summary
In some circumstances vnode may become null, and trying to access a property on that will rise a TypeError exception. here is a screenshot of the exception:

This rare issue happened in a workplace repo, hence I can't provide you the code; but If you guys think It is absolutely necessary to have a look by yourself, I can try to reproduce it in new repository ( I'd actually tried once, but it wasn't successful. )

Does this PR introduce a breaking change?
No breaking changes.

Other information

Please paste the results of preact info here.

Environment Info:
System:
OS: Windows 10 10.0.18362
CPU: (8) x64 Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz
Binaries:
Node: 14.15.4 - C:\Program Files\nodejs\node.EXE
Yarn: 1.21.1 - D:\Program Files (x86)\Yarn\bin\yarn.CMD
npm: 6.14.10 - C:\Program Files\nodejs\npm.CMD
Browsers:
Chrome: 88.0.4324.146
Edge: Spartan (44.18362.449.0)
npmPackages:
preact: ^10.3.2 => 10.5.7
preact-cli: ^3.0.0 => 3.0.3
preact-render-to-string: ^5.1.4 => 5.1.12
preact-router: ^3.2.1 => 3.2.1

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