fix textblock and texstpresenter issues related to password char.

src/Avalonia.Controls/Presenters/TextPresenter.cs

@@ -223,11 +223,11 @@ namespace Avalonia.Controls.Presenters
 
             if (PasswordChar != default(char))
             {
-                result = base.CreateFormattedText(constraint, new string(PasswordChar, Text.Length));
+                result = base.CreateFormattedText(constraint, new string(PasswordChar, text?.Length ?? 0));
             }
             else
             {
-                result = base.CreateFormattedText(constraint, Text);
+                result = base.CreateFormattedText(constraint, text);
             }
 
             var selectionStart = SelectionStart;

src/Avalonia.Controls/TextBlock.cs

@@ -111,6 +111,8 @@ namespace Avalonia.Controls
         /// </summary>
         public TextBlock()
         {
+            _text = string.Empty;
+
             Observable.Merge(
                 this.GetObservable(TextProperty).Select(_ => Unit.Default),
                 this.GetObservable(TextAlignmentProperty).Select(_ => Unit.Default),