Returns 'null' from resolve id hook rather than undefined to fix comp…

…atibility issue with newest Rollup version

package.json

@@ -44,34 +44,35 @@
 			"url": "https://github.com/wessberg",
 			"imageUrl": "https://avatars2.githubusercontent.com/u/20454213?s=460&v=4",
 			"role": "Lead Developer",
-			"twitter": "FredWessberg"
+			"twitter": "FredWessberg",
+			"github": "wessberg"
 		}
 	],
 	"license": "MIT",
 	"devDependencies": {
-		"core-js": "3.1.3",
+		"core-js": "3.1.4",
 		"@wessberg/scaffold": "1.0.18",
-		"@wessberg/ts-config": "0.0.40",
-		"@wessberg/rollup-plugin-ts": "1.1.54",
-		"rollup-plugin-node-resolve": "5.0.0",
-		"rollup": "1.12.4",
-		"rollup-pluginutils": "2.7.1",
-		"ava": "1.4.1",
+		"@wessberg/ts-config": "0.0.41",
+		"@wessberg/rollup-plugin-ts": "1.1.58",
+		"rollup-plugin-node-resolve": "5.0.3",
+		"rollup": "1.15.6",
+		"rollup-pluginutils": "2.8.1",
+		"ava": "2.1.0",
 		"standard-changelog": "2.0.11",
-		"ts-node": "8.2.0",
-		"tslint": "5.16.0",
-		"prettier": "1.17.1",
-		"pretty-quick": "1.11.0",
-		"husky": "2.3.0",
-		"np": "5.0.2"
+		"ts-node": "8.3.0",
+		"tslint": "5.17.0",
+		"prettier": "1.18.2",
+		"pretty-quick": "1.11.1",
+		"husky": "2.4.1",
+		"np": "5.0.3"
 	},
 	"dependencies": {
-		"@types/node": "12.0.3",
+		"@types/node": "12.0.8",
 		"@types/reserved-words": "^0.1.0",
 		"@types/resolve": "0.0.8",
 		"@wessberg/stringutil": "1.0.18",
 		"reserved-words": "^0.1.2",
-		"typescript": "3.5.1",
+		"typescript": "3.5.2",
 		"resolve": "1.11.0"
 	},
 	"main": "./dist/cjs/index.js",

test/setup/setup-rollup.ts

@@ -42,14 +42,14 @@ export async function generateRollupBundle(inputFiles: TestFile[] | TestFile, ro
 		throw new ReferenceError(`No entry could be found`);
 	}
 
-	const resolveId = (fileName: string, parent: string | null): string | undefined => {
+	const resolveId = (fileName: string, parent: string | undefined): string | null => {
 		const absolute = isAbsolute(fileName) ? fileName : join(parent == null ? "" : dirname(parent), fileName);
 		for (const ext of ["", ".ts", ".js", ".mjs"]) {
 			const withExtension = `${absolute}${ext}`;
 			const matchedFile = files.find(file => file.fileName === withExtension);
 			if (matchedFile != null) return withExtension;
 		}
-		return undefined;
+		return null;
 	};
 
 	const load = (id: string): string | null => {