Add check for _client existence in modbus (#52719)

.coveragerc

@@ -633,6 +633,7 @@ omit =
     homeassistant/components/mjpeg/camera.py
     homeassistant/components/mochad/*
     homeassistant/components/modbus/climate.py
+    homeassistant/components/modbus/modbus.py
     homeassistant/components/modem_callerid/sensor.py
     homeassistant/components/motion_blinds/__init__.py
     homeassistant/components/motion_blinds/const.py

homeassistant/components/modbus/modbus.py

@@ -310,6 +310,8 @@ class ModbusHub:
         """Convert async to sync pymodbus call."""
         if self._config_delay:
             return None
+        if not self._client:
+            return None
         if not self._client.is_socket_open():
             return None
         async with self._lock: