[babel-plugin] Warn/error for undefined `sel` instead of returning the empty string

[nolanlawson]

According to our unit tests, the empty string condition here is uncovered:

lwc/packages/@lwc/babel-plugin-component/src/component.ts

Line 52 in 1ca525a

const componentName = namespace && kebabCasedName ? `${namespace}-${kebabCasedName}` : '';

Defaulting to the empty string here is questionable because you will end up with an error at runtime here:

lwc/packages/@lwc/engine-core/src/framework/api.ts

Lines 676 to 681 in 1ca525a

	const sel = getComponentRegisteredName(Ctor);
	if (isUndefined(sel) || sel === '') {
	throw new Error(
	`Invalid LWC constructor ${toString(Ctor)} does not have a registered name`
	);
	}

This is hard to debug, and we should never have an undefined name/namespace in the Babel plugin, so it may be safer to throw an error or at least log a warning.