Fix boat value speed placeholder code

boatdata.py

@@ -233,7 +233,7 @@ class BoatValueSpeed(BoatValue):
     def format(self):
         if self.simulated:
             return "5.3"
-        if not self.value or not self.valid:
+        if (self.value is None) or not self.valid:
             return self.placeholder
         if self.value < 20:
             formatted = f"{self.value:3.1f}"