Do not create the default folders for DirectoryStore on Open (#2773)

For convenience, the DirectoryStore class creates the default folders when the DirectoryStore is opened. (since #2720)
However this may lead into issues mapping folders to the store when started in docker.

Fix: remove the code which creates default folders and ensure a null directory does not throw a NullReferenceException.

Stack/Opc.Ua.Core/Security/Certificates/DirectoryCertificateStore.cs

@@ -109,7 +109,7 @@ public void Open(string location, bool noPrivateKeys = false)
                     NoPrivateKeys = noPrivateKeys;
                     StorePath = location;
                     m_directory = directory;
-                    if (m_noSubDirs)
+                    if (m_noSubDirs || m_directory == null)
                     {
                         m_certificateSubdir = m_directory;
                         m_crlSubdir = m_directory;
@@ -125,39 +125,6 @@ public void Open(string location, bool noPrivateKeys = false)
                     // force load
                     ClearCertificates();
                     m_lastDirectoryCheck = DateTime.MinValue;
-
-                    // create folders if they do not exist
-                    try
-                    {
-                        if (!m_directory.Exists)
-                        {
-                            m_directory.Create();
-                        }
-
-                        if (!m_certificateSubdir.Exists)
-                        {
-                            m_certificateSubdir.Create();
-                        }
-
-                        if (noPrivateKeys)
-                        {
-                            if (!m_crlSubdir.Exists)
-                            {
-                                m_crlSubdir.Create();
-                            }
-                        }
-                        else
-                        {
-                            if (!m_privateKeySubdir.Exists)
-                            {
-                                m_privateKeySubdir.Create();
-                            }
-                        }
-                    }
-                    catch (IOException ex)
-                    {
-                        Utils.LogError("Failed to create certificate store: {0}", ex.Message);
-                    }
                 }
             }
         }
@@ -825,17 +792,16 @@ private IDictionary<string, Entry> Load(string thumbprint)
                 DateTime now = DateTime.UtcNow;
 
                 // refresh the directories.
-                m_certificateSubdir.Refresh();
+                m_certificateSubdir?.Refresh();
 
                 if (!NoPrivateKeys)
                 {
                     m_privateKeySubdir?.Refresh();
                 }
 
                 // check if store exists.
-                if (!m_certificateSubdir.Exists)
+                if (m_certificateSubdir?.Exists != true)
                 {
-                    m_certificateSubdir.Create();
                     ClearCertificates();
                     return m_certificates;
                 }