Fix the error message selection condition

[dead-claudia]

Description

The previous condition was basically "if this is non-nullish or a boolean". That "or a boolean" is very obviously redundant. In reality, the intent is it's supposed to be detecting holes.

I have no idea how I missed this mistake when I first wrote this.

Motivation and Context

See above.

How Has This Been Tested?

Untested locally, but it's a simple and obvious enough fix that I'm fine just letting CI test it. If tests somehow fail, I can patch the broken tests accordingly.

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 it already:
• I have read https://mithril.js.org/contributing.html.