add IsEmptyPart method to ensure stream is disposed

Author: mikeebowen

src/DocumentFormat.OpenXml.Framework/Packaging/OpenXmlPart.cs

@@ -448,6 +448,18 @@ internal virtual bool IsInVersion(FileFormatVersions version)
             return true;
         }
 
+        internal override bool IsEmptyPart()
+        {
+            bool isEmptyPart;
+
+            using (Stream stream = GetStream())
+            {
+                isEmptyPart = stream.Length == 0;
+            }
+
+            return isEmptyPart;
+        }
+
         #endregion
 
         #region internal methods

src/DocumentFormat.OpenXml.Framework/Packaging/OpenXmlPartContainer.cs

@@ -1657,6 +1657,11 @@ internal OpenXmlPart CreateOpenXmlPart(string relationshipType)
         // find all reachable parts from the package root, the dictionary also used for cycle reference defense
         internal abstract void FindAllReachableParts(IDictionary<OpenXmlPart, bool> reachableParts);
 
+        internal virtual bool IsEmptyPart()
+        {
+            return false;
+        }
+
         #endregion
 
         // Checks if the target part is in the same OpenXmlPackage as this part.

src/DocumentFormat.OpenXml.Framework/Validation/DocumentValidator.cs

@@ -107,8 +107,7 @@ private void ValidatePart(OpenXmlPart part, ValidationContext context)
                     {
                         Validate(context);
                     }
-                    else if (part.Uri.ToString().EndsWith(".xml", System.StringComparison.InvariantCultureIgnoreCase) &&
-                              part.GetStream().Length == 0)
+                    else if (part.Uri.ToString().EndsWith(".xml", System.StringComparison.InvariantCultureIgnoreCase) && part.IsEmptyPart())
                     {
                         context.AddError(new ValidationErrorInfo
                         {