MQTT - blank state_topic allowed in code but not configuration

### The problem

I have discovered that it is not possible to stop MQTT from publishing all sensor topics.

I am told that the C++ code allows for blank `state_topic` to prevent publishing per sensor, but the configuration validation prevents the blank topic.

https://github.com/esphome/esphome/blob/dev/esphome/components/mqtt/mqtt_component.cpp#L42-L46
https://github.com/esphome/esphome/blob/dev/esphome/config_validation.py#L1030-L1035

Could a blank `state_topic` be allowed to prevent the publishing by MQTT.

I am developing a custom component and only want to publish a custom payload to MQTT.

### Which version of ESPHome has the issue?

2022.9.2

### What type of installation are you using?

Home Assistant Add-on

### Which version of Home Assistant has the issue?

2022.9.7

### What platform are you using?

ESP8266

### Board

WEMOS D1 mini V3

### Component causing the issue

MQTT

### Example YAML snippet

```yaml
sensor:
- platform: emontx4
vrms:
id: iv
name: Vrms
retain: false
state_topic:
```

### Anything in the logs that might be useful for us?

_No response_

### Additional information

I also think that the automatic publishing of MQTT topics is something born out of the old mechanism for communicating with HA.

In a more diverse ecosystem, it should be possible to disable MQTT from publishing all these topics and flooding Brokers. A top level setting to disable all automatic publishing and then allow it to be enabled on a sensor by sensor basis.

So if Discovery disabled, allow as secondary setting to disable MQTT standard publishing.