chore(hazard_detail): Accept 0 as severity value

Author: Rup-Narayan-Rajbanshi

pystac_monty/extension.py

@@ -269,7 +269,7 @@ def __init__(
     ) -> None:
         self.properties = {}
 
-        if severity_value:
+        if severity_value is not None:
             self.severity_value = severity_value
         if severity_unit:
             self.severity_unit = severity_unit