fix: stop SeveritySlider from crashing if severity is null.

Migrates existing null severity threats to 'low'. While it shouldn't be set if action_item is not marked, null severity is still allowed, but it will be treated as if set to 'low'.

app/src/components/model/modals/SeveritySlider.js

@@ -35,7 +35,7 @@ export function SeveritySlider({ severity, onChange, ...props }) {
       onChangeCommitted={(value) => {
         onChange(value);
       }}
-      value={severity}
+      value={severity || "low"} // Default to low in the odd case where we get a null severity value, allowed by backend.
       {...props}
     />
   );

core/src/data/migrations/24_update_threats_with_no_severity.sql

@@ -0,0 +1,2 @@
+UPDATE threats
+SET severity = 'low' WHERE is_action_item = true and severity = null;