Cleaning up code by making vnode.attrs always non-null

[kfule]

Commit f9e5163 made vnode.attrs always non-null, so there is no need for code to make vnode.attrs null or assume vnode.attrs is null.

This pr is a re-opening of #2819.

Description

The following two processes have been removed

• Workaround for vnode of textarea when vnode.attrs is null (Edit: This workaround was aimed for assigning text to attrs.value. But assigning to attrs.value was removed as of the commit. So, this code is not needed now regardless of whether the attrs is null or not.)
• Processing of key removal to set vnode.attrs to null

For the latter, the behavior will change when there is only key in attrs (key is not removed from attrs). Some tests have also been changed to ensure that the changes are appropriate.

Motivation and Context

There is still a not very useful code that assumes that vnode.attrs will be null. Removing that code will reduce the amount of code and improve the outlook of the process.

How Has This Been Tested?

Running "npm run test"

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My change requires a documentation update, and I've opened a pull request to update them already: https://github.com/MithrilJS/docs/pulls/NNNN
• I have read https://mithril.js.org/contributing.html.