Fix completely empty table cells causing Word errors

OpenXmlPowerTools.Tests/DocumentAssemblerTests.cs

@@ -107,6 +107,7 @@ public class DaTests
         [InlineData("DA264-InvalidRunLevelRepeat.docx", "DA-Data.xml", true)]
         [InlineData("DA265-RunLevelRepeatWithWhiteSpaceBefore.docx", "DA-Data.xml", false)]
         [InlineData("DA266-RunLevelRepeat-NoData.docx", "DA-Data.xml", true)]
+        [InlineData("DA268-Block-Conditional-In-Table-Cell.docx", "DA268-data.xml", false)]
         public void DA101(string name, string data, bool err)
         {
             var sourceDir = new DirectoryInfo("../../../../TestFiles/");

OpenXmlPowerTools/DocumentAssembler/DocumentAssembler.cs

@@ -873,9 +873,24 @@ private class RunReplacementInfo
                     }
                     return null;
                 }
+                var transformedNodes = element.Nodes().Select(n => ContentReplacementTransform(n, data, templateError, owningPart));
+                if (element.Name == W.tc)
+                {
+                    // Check if the table cell contains any block-level elements
+                    // Valid block-level elements in a table cell: p (paragraph), tbl (table), sdt (structured document tag), customXml
+                    var nodesList = transformedNodes.ToList();
+                    var hasBlockLevelContent = nodesList.Any(n => n is XElement xe && 
+                        (xe.Name == W.p || xe.Name == W.tbl || xe.Name == W.sdt || xe.Name == W.customXml));
+                    if (!hasBlockLevelContent)
+                    {
+                        // Table cells must contain at least one block-level element -- add an empty paragraph
+                        nodesList.Add(new XElement(W.p));
+                    }
+                    transformedNodes = nodesList;
+                }
                 return new XElement(element.Name,
                     element.Attributes(),
-                    element.Nodes().Select(n => ContentReplacementTransform(n, data, templateError, owningPart)));
+                    transformedNodes);
             }
             return node;
         }

TestFiles/DA268-data.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Dummy>
+  <a>False</a>
+</Dummy>