fixed a problem with asserts trying to merge empty aggregate transactions.

if the aggregate transaction is empty, don't execute it.  It's not technically
illegal, but it's a waste, and it causes the assert.

editor/base/nsEditor.cpp

@@ -1481,7 +1481,20 @@ NS_IMETHODIMP nsEditor::InsertTextImpl(const nsString& aStringToInsert)
   }
   else if (NS_ERROR_EDITOR_NO_TEXTNODE==result) 
   {
-    result = Do(aggTxn);
+
+    // only execute the aggTxn if we actually populated it with at least one sub-txn
+    PRInt32 count=0;
+    aggTxn->GetCount(&count);
+    if (0!=count)
+    {
+      result = Do(aggTxn);
+    }
+    else
+    {
+      result = NS_OK;
+    }
+
+    // create the text node
     if (NS_SUCCEEDED(result))
     {
       nsCOMPtr<nsIDOMSelection> selection;

editor/libeditor/base/nsEditor.cpp

@@ -1481,7 +1481,20 @@ NS_IMETHODIMP nsEditor::InsertTextImpl(const nsString& aStringToInsert)
   }
   else if (NS_ERROR_EDITOR_NO_TEXTNODE==result) 
   {
-    result = Do(aggTxn);
+
+    // only execute the aggTxn if we actually populated it with at least one sub-txn
+    PRInt32 count=0;
+    aggTxn->GetCount(&count);
+    if (0!=count)
+    {
+      result = Do(aggTxn);
+    }
+    else
+    {
+      result = NS_OK;
+    }
+
+    // create the text node
     if (NS_SUCCEEDED(result))
     {
       nsCOMPtr<nsIDOMSelection> selection;