Make sure KEM implementations don't return an empty shared secret

If `set_public_key` is never called we won't have a shared secret to
return.  This aligns the implementations with the one in the openssl
plugin.

src/libstrongswan/plugins/botan/botan_kem.c

@@ -281,8 +281,12 @@ METHOD(key_exchange_t, set_public_key, bool,
 METHOD(key_exchange_t, get_shared_secret, bool,
 	private_key_exchange_t *this, chunk_t *secret)
 {
-	*secret = chunk_clone(this->shared_secret);
-	return TRUE;
+	if (this->shared_secret.ptr != NULL)
+	{
+		*secret = chunk_clone(this->shared_secret);
+		return TRUE;
+	}
+	return FALSE;
 }
 
 METHOD(key_exchange_t, get_method, key_exchange_method_t,

src/libstrongswan/plugins/ml/ml_kem.c

@@ -942,8 +942,12 @@ METHOD(key_exchange_t, get_method, key_exchange_method_t,
 METHOD(key_exchange_t, get_shared_secret, bool,
 	private_key_exchange_t *this, chunk_t *secret)
 {
-	*secret = chunk_clone(this->shared_secret);
-	return TRUE;
+	if (this->shared_secret.ptr != NULL)
+	{
+		*secret = chunk_clone(this->shared_secret);
+		return TRUE;
+	}
+	return FALSE;
 }
 
 #ifdef TESTABLE_KE

src/libstrongswan/plugins/wolfssl/wolfssl_kem.c

@@ -241,8 +241,12 @@ METHOD(key_exchange_t, set_public_key, bool,
 METHOD(key_exchange_t, get_shared_secret, bool,
 	private_key_exchange_t *this, chunk_t *secret)
 {
-	*secret = chunk_clone(this->shared_secret);
-	return TRUE;
+	if (this->shared_secret.ptr != NULL)
+	{
+		*secret = chunk_clone(this->shared_secret);
+		return TRUE;
+	}
+	return FALSE;
 }
 
 METHOD(key_exchange_t, get_method, key_exchange_method_t,