fix empty TOPIC_BASE issue (#21740)

* fix empty TOPIC_BASE issue

if the value of the TOPIC_BASE is empty then we need to remove "~" from the topic value if it exists. 
by doing `if base:` on line 239 the condition will be false if the value is empty so the '~' will not be stripped from the topic value.
I simply removed the `if base:` line and added `if TOPIC_BASE in payload:`

* Update homeassistant/components/mqtt/discovery.py

Co-Authored-By: engrbm87 <engrbm87@gmail.com>

homeassistant/components/mqtt/discovery.py

@@ -235,8 +235,8 @@ async def async_device_message_received(topic, payload, qos):
                 key = DEVICE_ABBREVIATIONS.get(key, key)
                 device[key] = device.pop(abbreviated_key)
 
-        base = payload.pop(TOPIC_BASE, None)
-        if base:
+        if TOPIC_BASE in payload:
+            base = payload.pop(TOPIC_BASE)
             for key, value in payload.items():
                 if isinstance(value, str) and value:
                     if value[0] == TOPIC_BASE and key.endswith('_topic'):