Fix up dsa.ImportParameters method for Ubuntu runs

It seems to be something about some updates in the OS.  Maybe OpenSSL changes or OS changes.  It broke on the Build server and locally on my Ubuntu 22.04 that I recently updated.

After hours of debugging and finally getting the debugger to attach to the test I could narrow down where the error was happening.  Copilot helped me to find a solution.  I included the explanation here:

Certainly! The line parameters.Y = key.Parameters.G.ModPow(key.X, key.Parameters.P).ToByteArrayUnsigned(); is used to derive the public key component Y from the private key components in the DSA (Digital Signature Algorithm) parameters. Here's a detailed explanation:
DSA Key Components
•	P: A large prime number.
•	Q: A prime divisor of P-1.
•	G: A generator of a subgroup of order Q in the multiplicative group of integers modulo P.
•	X: The private key, which is a randomly chosen integer such that 0 < X < Q.
•	Y: The public key, which is derived from the private key.
Deriving the Public Key Y
In DSA, the public key Y is derived from the private key X using the following formula: [ Y = G^X \mod P ] Where:
•	G is the generator.
•	X is the private key.
•	P is the prime modulus.
Explanation of the Code
•	key.Parameters.G: This is the generator G.
•	key.X: This is the private key X.
•	key.Parameters.P: This is the prime modulus P.
The method ModPow is used to perform modular exponentiation, which calculates ( G^X \mod P ). This operation is efficient and secure for large numbers, which are typical in cryptographic algorithms.
•	key.Parameters.G.ModPow(key.X, key.Parameters.P): This calculates ( G^X \mod P ), resulting in the public key Y as a BigInteger.
•	.ToByteArrayUnsigned(): This converts the resulting BigInteger to an unsigned byte array, which is the format required by the DSAParameters structure.

Author: JoeShook

MimeKit/Cryptography/AsymmetricAlgorithmExtensions.cs

@@ -247,8 +247,12 @@ static AsymmetricAlgorithm GetAsymmetricAlgorithm (DsaPrivateKeyParameters key,
 			var parameters = GetDSAParameters (key);
 			parameters.X = GetPaddedByteArray (key.X, parameters.Q.Length);
 
-			if (pub != null)
+			if (pub != null) {
 				parameters.Y = GetPaddedByteArray (pub.Y, parameters.P.Length);
+			} else {
+				// If pub is null, derive Y from the private key parameters
+				parameters.Y = key.Parameters.G.ModPow (key.X, key.Parameters.P).ToByteArrayUnsigned ();
+			}
 
 			var dsa = new DSACryptoServiceProvider ();