transform/transformBundle can return null/undefined

I did not find it explicitly in documentation, but here is the code:
https://github.com/rollup/rollup/blob/master/src/utils/transform.js#L78
https://github.com/rollup/rollup/blob/master/src/utils/transformBundle.js#L12

types/rollup/index.d.ts

@@ -137,9 +137,9 @@ export interface Plugin {
 	 */
 	resolveId?(importee: string, importer: string | undefined): string | null | undefined | false | 0 | ''
 	/** A module transformer function */
-	transform?(this: TransformContext, source: string, id: string): string | { code: string, map: SourceMap }
+	transform?(this: TransformContext, source: string, id: string): string | null | undefined | { code: string, map: SourceMap }
 	/** A bundle transformer function */
-	transformBundle?(source: string, options: { format: Format }): string | { code: string, map: SourceMap }
+	transformBundle?(source: string, options: { format: Format }): string | null | undefined | { code: string, map: SourceMap }
 	/** Function hook called when bundle.generate() is being executed. */
 	ongenerate?(options: GenerateOptions, bundle: Bundle): void
 	/** Function hook called when bundle.write() is being executed, after the file has been written to disk. */

types/rollup/rollup-tests.ts

@@ -14,12 +14,23 @@ const plugin: Plugin = {
     transform(source, id) {
         if (id === 'rxjs') {
             this.error(new Error(`Don't import this directly`))
+            return null
         }
         const indexOfQuote = source.indexOf('"')
         if (indexOfQuote >= 0) {
             this.warn(`Prefer ' over " for strings`, indexOfQuote)
+            return undefined
         }
         return source
+    },
+    transformBundle(source, options) {
+        if (options.format === 'iife') {
+            return `window.nonModule = true\n${source}`
+        } else if (options.format === 'cjs') {
+            return null
+        }
+
+        return undefined
     }
 }