An SCM-only user has no groups, so do not throw an undocumented exception looking them up.

… hudson.ExpressionFactory2$JexlExpression evaluate
WARNING: Caught exception evaluating: it.authorities in /user/demo/. Reason: java.lang.reflect.InvocationTargetException
java.lang.reflect.InvocationTargetException
	at …
Caused by: org.acegisecurity.userdetails.UsernameNotFoundException: demo
	at hudson.security.SecurityRealm$None$2.loadUserByUsername(SecurityRealm.java:494)
	at jenkins.security.ImpersonatingUserDetailsService.loadUserByUsername(ImpersonatingUserDetailsService.java:32)
	at hudson.model.User.impersonate(User.java:282)
	at hudson.model.User.getAuthorities(User.java:769)
	... 93 more

core/src/main/java/hudson/model/User.java

@@ -802,7 +802,14 @@ public class User extends AbstractModelObject implements AccessControlled, Descr
             return Collections.emptyList();
         }
         List<String> r = new ArrayList<String>();
-        for (GrantedAuthority a : impersonate().getAuthorities()) {
+        Authentication authentication;
+        try {
+            authentication = impersonate();
+        } catch (UsernameNotFoundException x) {
+            LOGGER.log(Level.FINE, "cannot look up authorities for " + id, x);
+            return Collections.emptyList();
+        }
+        for (GrantedAuthority a : authentication.getAuthorities()) {
             if (a.equals(SecurityRealm.AUTHENTICATED_AUTHORITY)) {
                 continue;
             }