Fix className types in engine-core

[nolanlawson]

If you set a breakpoint in this function:

lwc/packages/@lwc/engine-core/src/framework/modules/computed-class-attr.ts

Line 23 in 3665f31

export function getMapFromClassName(className: string | undefined): Record<string, boolean> {

and run the Karma tests, you'll find that className can be a string, null, undefined, a number... Lots of things. But TypeScript thinks it can only be a string or undefined. We should fix these types to reflect reality.