fix: return empty array when no multiaddrs are known

Returning `undefined` makes the address length check in [dialler/index.js](https://github.com/libp2p/js-libp2p/blob/master/src/dialer/index.js#L73-L75)
fail with `cannot read property length of undefined` so the change
here is to always return an array, but it might be empty if we don't
know any multiaddrs for the given peer.

MIGRATION_TEMPLATE.md

@@ -0,0 +1,45 @@
+<!--Specify versions for migration below-->
+# Migrating to libp2p@__
+
+A migration guide for refactoring your application code from libp2p v__ to v__.
+
+## Table of Contents
+
+- [API](#api)
+- [Module Updates](#module-updates)
+
+## API
+
+<!--Describe breaking APIs with examples for Before and After
+Example:
+
+### Peer Discovery
+
+__Describe__
+
+**Before**
+
+```js
+
+```
+
+**After**
+
+```js
+
+```
+
+-->
+
+## Module Updates
+
+With this release you should update the following libp2p modules if you are relying on them:
+
+<!--Specify module versions in JSON for migration below.
+It's recommended to check package.json changes for this: 
+`git diff <release> <prev> -- package.json`
+-->
+
+```json
+
+```

src/peer-store/address-book.js

@@ -179,7 +179,7 @@ class AddressBook extends Book {
     const record = this.data.get(peerId.toB58String())
 
     if (!record) {
-      return undefined
+      return []
     }
 
     return record.map((address) => {

test/peer-store/address-book.spec.js

@@ -323,10 +323,10 @@ describe('addressBook', () => {
       throw new Error('invalid peerId should throw error')
     })
 
-    it('returns undefined if no multiaddrs are known for the provided peer', () => {
+    it('returns empty array if no multiaddrs are known for the provided peer', () => {
       const addresses = ab.getMultiaddrsForPeer(peerId)
 
-      expect(addresses).to.not.exist()
+      expect(addresses).to.be.empty()
     })
 
     it('returns the multiaddrs stored', () => {