control: set_parameter_from_group: handle no max

Handle the case where no members of the group have a value (i.e. they
aren't  on D-Bus).  In that case, remove that parameter from the
manager.

Change-Id: Idc70d637b1031288fa9bf3a8f23cd7fca09ae8fb
diff --git a/control/json/actions/set_parameter_from_group_max.cpp b/control/json/actions/set_parameter_from_group_max.cpp
index 622c20c..42b5c3c 100644
--- a/control/json/actions/set_parameter_from_group_max.cpp
+++ b/control/json/actions/set_parameter_from_group_max.cpp
@@ -96,7 +96,7 @@
         }
     }
 
-    if (_modifier)
+    if (_modifier && max)
     {
         try
         {
@@ -112,7 +112,14 @@
         }
     }
 
-    Manager::setParameter(_name, *max);
+    if (max)
+    {
+        Manager::setParameter(_name, *max);
+    }
+    else
+    {
+        Manager::deleteParameter(_name);
+    }
 }
 
 void SetParameterFromGroupMax::setParameterName(const json& jsonObj)