fix: Update MarkdownHeaderTextSplitter.cs (#485)

This change prevents errors when the markdown file includes lines that start with # but have no following characters.

Co-authored-by: Konstantin S. <[email protected]>

src/Splitters/Abstractions/src/Text/MarkdownHeaderTextSplitter.cs

@@ -122,6 +122,8 @@ private bool IsHeader(string line, out int len)
         len = 0;
         foreach (var header in _headersToSplitOn)
         {
+            if (line.Length <= header.Length + 1)
+                return false;//Empty lines starting with #s should not be considered as headers. Removing this line would result in exceptions in that conditions
             if (line.Trim().StartsWith(header, StringComparison.Ordinal) && line[header.Length] == ' ')
             {
                 len = header.Length;
@@ -131,4 +133,4 @@ private bool IsHeader(string line, out int len)
 
         return false;
     }
-}
+}