[fix] Fix misleading error message

Use the correct error message if the server sends an empty subprotocol
name.

lib/websocket.js

@@ -771,7 +771,7 @@ function initAsClient(websocket, address, protocols, options) {
     const serverProt = res.headers['sec-websocket-protocol'];
     let protError;
 
-    if (serverProt) {
+    if (serverProt !== undefined) {
       if (!protocolSet.size) {
         protError = 'Server sent a subprotocol but none was requested';
       } else if (!protocolSet.has(serverProt)) {

test/websocket.test.js

@@ -895,7 +895,7 @@ describe('WebSocket', () => {
       });
     });
 
-    it('fails if server sends an invalid subprotocol', (done) => {
+    it('fails if server sends an invalid subprotocol (1/2)', (done) => {
       const wss = new WebSocket.Server({
         handleProtocols: () => 'baz',
         server
@@ -914,6 +914,36 @@ describe('WebSocket', () => {
       });
     });
 
+    it('fails if server sends an invalid subprotocol (2/2)', (done) => {
+      server.once('upgrade', (req, socket) => {
+        const key = crypto
+          .createHash('sha1')
+          .update(req.headers['sec-websocket-key'] + GUID)
+          .digest('base64');
+
+        socket.end(
+          'HTTP/1.1 101 Switching Protocols\r\n' +
+            'Upgrade: websocket\r\n' +
+            'Connection: Upgrade\r\n' +
+            `Sec-WebSocket-Accept: ${key}\r\n` +
+            'Sec-WebSocket-Protocol:\r\n' +
+            '\r\n'
+        );
+      });
+
+      const ws = new WebSocket(`ws://localhost:${server.address().port}`, [
+        'foo',
+        'bar'
+      ]);
+
+      ws.on('open', () => done(new Error("Unexpected 'open' event")));
+      ws.on('error', (err) => {
+        assert.ok(err instanceof Error);
+        assert.strictEqual(err.message, 'Server sent an invalid subprotocol');
+        ws.on('close', () => done());
+      });
+    });
+
     it('fails if server sends no subprotocol', (done) => {
       const wss = new WebSocket.Server({
         handleProtocols() {},