FloatingHint is can't Float in PasswordBox when password is only SPACE. fixes #574

Author: Chi_Xiaohui

MaterialDesignThemes.Wpf/HintProxyFabric.PasswordBox.cs

@@ -9,7 +9,7 @@ private sealed class PasswordBoxHintProxy : IHintProxy
         {
             private readonly PasswordBox _passwordBox;
 
-            public bool IsEmpty() => string.IsNullOrWhiteSpace(_passwordBox.Password);          
+            public bool IsEmpty() => string.IsNullOrEmpty(_passwordBox.Password);
 
             public object Content => _passwordBox.Password;