If there are no implicit elements, do not clone

This fixes unit test failures that arose because cloning the XmlDocument
caused a spurious `\n` character to be inserted after the XML declaration
tag in the saved output.

src/XMakeBuildEngine/Construction/ProjectRootElement.cs

@@ -157,6 +157,11 @@ public class ProjectRootElement : ProjectElementContainer
         /// </summary>
         private BuildEventContext _buildEventContext;
 
+        /// <summary>
+        /// Xpath expression that will find any element with the implicit attribute
+        /// </summary>
+        private static readonly string ImplicitAttributeXpath = $"//*[@{XMakeAttributes.@implicit}]";
+
         /// <summary>
         /// Initialize a ProjectRootElement instance from a XmlReader.
         /// May throw InvalidProjectFileException.
@@ -1781,15 +1786,21 @@ public void Save(Encoding saveEncoding)
 
         private XmlDocument RemoveImplicits()
         {
+            if (XmlDocument.SelectSingleNode(ImplicitAttributeXpath) == null)
+            {
+                return XmlDocument;
+            }
+
             var xmlWithNoImplicits = (XmlDocument) XmlDocument.CloneNode(deep: true);
 
             var implicitElements =
-                xmlWithNoImplicits.SelectNodes($"//*[@{XMakeAttributes.@implicit}]");
+                xmlWithNoImplicits.SelectNodes(ImplicitAttributeXpath);
 
             foreach (XmlNode implicitElement in implicitElements)
             {
                 implicitElement.ParentNode.RemoveChild(implicitElement);
             }
+
             return xmlWithNoImplicits;
         }