move file extension check to IsEmptyPartMethod

Author: mikeebowen

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

@@ -450,6 +450,11 @@ internal virtual bool IsInVersion(FileFormatVersions version)
 
         internal override bool IsEmptyPart()
         {
+            if (!Uri.ToString().EndsWith(".xml", System.StringComparison.InvariantCultureIgnoreCase))
+            {
+                return false;
+            }
+
             using (Stream stream = GetStream())
             {
                 return stream.Length == 0;

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

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