Can't emit package with empty binary field, ver 5.1.1

[ForJobsGit]

Describe the bug
Can't emit package with empty binary field

To Reproduce

requirements.txt

gunicorn[gevent]==20.1.0
gevent-websocket==0.10.1
greenlet==1.1.2
Flask==2.0.2
Werkzeug==2.0.2
Jinja2==3.0.2
Flask-SocketIO==5.1.1
python-socketio[client]==5.4.1

server.py

import gevent.monkey
gevent.monkey.patch_all()

from datetime import datetime

from flask import Flask
from flask_socketio import SocketIO

flask_server = Flask(__name__)
flask_server.config['SECRET_KEY'] = 'test'

socket_io = SocketIO(flask_server, logger=True, engineio_logger=True)


@socket_io.on('upload_data')
def upload(*args):
    print(f'{datetime.utcnow()} upload_data {args}')

client.py

from datetime import datetime
import time
import socketio

sio = socketio.Client()
sio.connect(url='http://localhost:8080')

sio.emit(event='upload_data', data={'emit_at': str(datetime.utcnow()), 'binary': b''})

run

gunicorn  --bind=127.0.0.1:8080 --workers=1 --worker-class=geventwebsocket.gunicorn.workers.GeventWebSocketWorker server:flask_server

server logs

WHJ3PDQvATsZZBJdAAAC: Sending packet OPEN data {'sid': 'WHJ3PDQvATsZZBJdAAAC', 'upgrades': ['websocket'], 'pingTimeout': 20000, 'pingInterval': 25000}
WHJ3PDQvATsZZBJdAAAC: Received request to upgrade to websocket
WHJ3PDQvATsZZBJdAAAC: Upgrade to websocket successful
WHJ3PDQvATsZZBJdAAAC: Received packet MESSAGE data 0
WHJ3PDQvATsZZBJdAAAC: Sending packet MESSAGE data 0{"sid":"Yg88TtqALWtXfJxZAAAD"}
WHJ3PDQvATsZZBJdAAAC: Received packet MESSAGE data 51-["upload_data",{"emit_at":"2021-10-20 18:27:29.371042","binary":{"_placeholder":true,"num":0}}]
WHJ3PDQvATsZZBJdAAAC: Received packet NOOP data None

[ForJobsGit]

Describe the bug
Can't emit package with empty binary field.

To Reproduce

requirements.txt

gunicorn[eventlet]==20.1.0
eventlet==0.30.2 # https://github.com/benoitc/gunicorn/pull/2581
greenlet==1.1.2
Flask==2.0.2
Werkzeug==2.0.2
Jinja2==3.0.2
Flask-SocketIO==5.1.1
python-socketio[client]==5.4.1

server.py

import eventlet
eventlet.monkey_patch()

from datetime import datetime

from flask import Flask
from flask_socketio import SocketIO

flask_server = Flask(__name__)
flask_server.config['SECRET_KEY'] = 'test'

socket_io = SocketIO(flask_server, logger=True, engineio_logger=True)


@socket_io.on('upload_data')
def upload(*args):
    print(f'{datetime.utcnow()} upload_data {args}')

client.py

from datetime import datetime
import time
import socketio

sio = socketio.Client()
sio.connect(url='http://localhost:8080')

sio.emit(event='upload_data', data={'emit_at': str(datetime.utcnow()), 'binary': b''})

run

gunicorn  --bind=127.0.0.1:8080 --workers=1 --worker-class=eventlet server:flask_server

server logs

qtQxmkax4c0mK-XxAAAC: Sending packet OPEN data {'sid': 'qtQxmkax4c0mK-XxAAAC', 'upgrades': ['websocket'], 'pingTimeout': 20000, 'pingInterval': 25000}
qtQxmkax4c0mK-XxAAAC: Received request to upgrade to websocket
qtQxmkax4c0mK-XxAAAC: Upgrade to websocket successful
qtQxmkax4c0mK-XxAAAC: Received packet MESSAGE data 0
qtQxmkax4c0mK-XxAAAC: Sending packet MESSAGE data 0{"sid":"49UFfA77lsFQjxp7AAAD"}
qtQxmkax4c0mK-XxAAAC: Received packet MESSAGE data 51-["upload_data",{"emit_at":"2021-10-20 22:27:49.703361","binary":{"_placeholder":true,"num":0}}]
qtQxmkax4c0mK-XxAAAC: Received packet NOOP data None