fix: add null check in libp2p.hangUp()

test: add hangup test and fix linting

src/index.js

@@ -311,10 +311,17 @@ class Libp2p extends EventEmitter {
    * @param {PeerInfo|PeerId|multiaddr|string} peer the peer to close connections to
    * @returns {Promise<void>}
    */
-  hangUp (peer) {
+  async hangUp (peer) {
     const peerInfo = getPeerInfo(peer, this.peerStore)
-    return Promise.all(
+
-      this.registrar.connections.get(peerInfo.id.toB58String()).map(connection => {
+    const connections = this.registrar.connections.get(peerInfo.id.toB58String())
+
+    if (!connections) {
+      return
+    }
+
+    await Promise.all(
+      connections.map(connection => {
         return connection.close()
       })
     )

test/dialing/direct.spec.js

@@ -371,6 +371,19 @@ describe('Dialing (direct, WebSockets)', () => {
       expect(connection.stat.timeline.close).to.exist()
     })
 
+    it('should be able to use hangup when no connection exists', async () => {
+      libp2p = new Libp2p({
+        peerInfo,
+        modules: {
+          transport: [Transport],
+          streamMuxer: [Muxer],
+          connEncryption: [Crypto]
+        }
+      })
+
+      await libp2p.hangUp(remoteAddr)
+    })
+
     it('should abort pending dials on stop', async () => {
       libp2p = new Libp2p({
         peerInfo,