Change Book.coverUrl to return null when there is no cover

CoverUrl now returns an undefined when there is no cover. However not
having a cover is expected behavior for an epub without a cover.

documentation/md/API.md

@@ -134,9 +134,9 @@ Set headers request should use
 
 ### coverUrl
 
-Get the cover url
+Get the cover url if there is a cover
 
-Returns **[Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise)<[string](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String)>** Promise resolves with url string
+Returns **[Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise)<[?string](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String)>** Promise resolves with maybe a url string
 
 ### getRange
 

src/book.js

@@ -661,19 +661,20 @@ class Book {
 
 	/**
 	 * Get the cover url
-	 * @return {Promise<string>} coverUrl
+	 * @return {Promise<?string>} coverUrl
 	 */
 	coverUrl() {
-		var retrieved = this.loaded.cover.
-			then(() => {
-				if(this.archived && this.cover) {
-					return this.archive.createUrl(this.cover);
-				}else{
-					return this.cover;
-				}
-			});
+		return this.loaded.cover.then(() => {
+			if (!this.cover) {
+				return null;
+			}
 
-		return retrieved;
+			if (this.archived) {
+				return this.archive.createUrl(this.cover);
+			} else {
+				return this.cover;
+			}
+		});
 	}
 
 	/**

test/book.js

@@ -61,7 +61,7 @@ describe('Book', function() {
 		});
 		it('should have a empty coverUrl', async function() {
 			let coverUrl = await book.coverUrl()
-			assert.equal(coverUrl, undefined, "cover url should be undefined" );
+			assert.equal(coverUrl, null, "cover url should be null" );
 		});
 	});
 });

types/book.d.ts

@@ -65,7 +65,7 @@ export default class Book {
 
     canonical(path: string): string;
 
-    coverUrl(): Promise<string>;
+    coverUrl(): Promise<string | null>;
 
     destroy(): void;