Treat null/missing CmpContext and IsMasterPage as equivalent in GetGxObject

HttpAjaxContext.GetGxObject(JArray, CmpContext, IsMasterPage) iterates the
array looking for an entry that matches the (CmpContext, IsMasterPage)
pair. The previous lookup called obj['CmpContext'].ToString() and
obj['IsMasterPage'].ToString() directly — when the array entry didn't
carry those keys, the indexer returned null and ToString() threw
NullReferenceException, which fell into the catch and made the function
return null, breaking the callers (ajax_rsp_assign_attri and similar).

Make the comparison null-safe and treat 'missing/null/empty' as
semantically equivalent to the default value:
- CmpContext: a missing/empty entry matches a null-or-empty parameter.
- IsMasterPage: a missing/empty entry matches IsMasterPage == false
  (default for bool). Comparison is case-insensitive against
  IsMasterPage.ToString().

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: claudiamurialdo

dotnet/src/dotnetframework/GxClasses/Core/Web/HttpAjaxContext.cs

@@ -286,7 +286,17 @@ private JObject GetGxObject(JArray array, String CmpContext, bool IsMasterPage)
 				for (int i = 0; i < array.Count; i++)
                 {
 					obj = array.GetObject(i);
-                    if (obj["CmpContext"].ToString().Equals(CmpContext) && obj["IsMasterPage"].ToString().Equals(IsMasterPage.ToString()))
+					string objCmpContext = obj["CmpContext"]?.ToString();
+					string objIsMasterPage = obj["IsMasterPage"]?.ToString();
+					// A missing/null/empty CmpContext on the array entry is equivalent to a null-or-empty parameter.
+					bool cmpContextMatches = string.IsNullOrEmpty(objCmpContext)
+						? string.IsNullOrEmpty(CmpContext)
+						: objCmpContext.Equals(CmpContext);
+					// A missing IsMasterPage on the array entry is treated as the default value (false).
+					bool isMasterPageMatches = string.IsNullOrEmpty(objIsMasterPage)
+						? !IsMasterPage
+						: objIsMasterPage.Equals(IsMasterPage.ToString(), StringComparison.OrdinalIgnoreCase);
+					if (cmpContextMatches && isMasterPageMatches)
                     {
                         return obj;
                     }
@@ -303,7 +313,7 @@ private JObject GetGxObject(JArray array, String CmpContext, bool IsMasterPage)
 			}
 
 			return null;
-        }        
+        }
 
         public void ajax_rsp_assign_attri(String CmpContext, bool IsMasterPage, String AttName, Object AttValue)
 		{