Add input validation to EncryptPassword method

Added a null or empty input check in the `EncryptPassword`
method to validate the `plainText` parameter before
proceeding with encryption logic. This ensures robustness
and prevents errors or unintended behavior when invalid
input is provided.

Author: michaelmsonne

src/SignToolGUI/Class/SecurePasswordManager.cs

@@ -79,6 +79,7 @@ private static string GenerateMachineSpecificKey()
         /// <returns>Base64 encoded encrypted password with metadata</returns>
         public static string EncryptPassword(string plainText)
         {
+            // Check for null or empty input
             if (string.IsNullOrEmpty(plainText))
                 return string.Empty;