Plugin resolve foo in 0.48.2 but fail in 0.49.0

[CodeTroopers]

This plugin can't resolve foo with rollup 0.49.0 althought it works with 0.48.2

{
	resolveId: function (id, importer) {
		let modulePath = /foo/.test(id) ? "./foo.js" : undefined;
		if (modulePath)
			return "./foo.js";
				
		// if no path was found, null must be returned to keep the plugin chain!
		return null;
	}
}

You can find the repro here

[CodeTroopers]

Resolved with 0.49.1