Fixed parameter check newValue for Replace to be only checked for null. Having an empty string is allowed as replacement value.

DocX/Container.cs

 
         public virtual void ReplaceText(string oldValue, string newValue, bool trackChanges = false, RegexOptions options = RegexOptions.None, Formatting newFormatting = null, Formatting matchFormatting = null, MatchFormattingOptions fo = MatchFormattingOptions.SubsetMatch)
         {
-            // PATCH BY HeDo - Arguments check!
             if (oldValue == null || oldValue.Length == 0)
                 throw new ArgumentException("oldValue cannot be null or empty", "oldValue");
 
-            if (newValue == null || newValue.Length == 0)
+            if (newValue == null)
                 throw new ArgumentException("newValue cannot be null or empty", "newValue");
             // ReplaceText in Headers of the document.
             Headers headers = Document.Headers;