MQTT module: Hardcoded leading slash in topic construction creates empty root level

[lincwill89-sudo]

Observed Behavior:
With the following configuration: mqtt_basetopic baresip/505
Baresip logs show: mqtt: Publishing on /baresip/505/event, subscribing to /baresip/505/command/+
Expected Behavior:
The leading slash should be omitted if not present in the config, resulting in:
mqtt: Publishing on baresip/505/event, subscribing to baresip/505/command/+