Add comments.

src/Workspaces/Core/Desktop/Workspace/SQLite/SQLitePersistentStorage_BulkPopulateIds.cs

@@ -202,6 +202,9 @@ string GetDocumentIdString(Document document)
 
             void AddIfUnknownId(string value, HashSet<string> stringsToAdd)
             {
+                // Null strings are not supported at all.  Just ignore these. Any read/writes 
+                // to null values will fail and will return 'false/null' to indicate failure
+                // (which is part of the documented contract of the persistence layer API).
                 if (value == null)
                 {
                     return;

src/Workspaces/Core/Desktop/Workspace/SQLite/SQLitePersistentStorage_StringIds.cs

@@ -34,6 +34,9 @@ private void FetchStringTable(SqlConnection connection)
 
         private int? TryGetStringId(SqlConnection connection, string value)
         {
+            // Null strings are not supported at all.  Just ignore these. Any read/writes 
+            // to null values will fail and will return 'false/null' to indicate failure
+            // (which is part of the documented contract of the persistence layer API).
             if (value == null)
             {
                 return null;