improve typings

Author: Yankovsky

src/utils/getOriginal.ts

@@ -1,6 +1,6 @@
 import { BreakpointSource } from '../types';
 
-export const getOriginal = (source: BreakpointSource): string | undefined => {
+export const getOriginal = (source: BreakpointSource) => {
   const srcSet = source.srcSets[source.srcSets.length - 1].srcSet;
 
   return srcSet['3x'] || srcSet['2x'] || srcSet['1x'];