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

@@ -280,10 +280,14 @@ METHOD(key_exchange_t, set_public_key, bool,
 
 METHOD(key_exchange_t, get_shared_secret, bool,
 	private_key_exchange_t *this, chunk_t *secret)
+{
+	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,
 	private_key_exchange_t *this)

src/libstrongswan/plugins/ml/ml_kem.c

@@ -941,10 +941,14 @@ 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)
+{
+	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

@@ -240,10 +240,14 @@ METHOD(key_exchange_t, set_public_key, bool,
 
 METHOD(key_exchange_t, get_shared_secret, bool,
 	private_key_exchange_t *this, chunk_t *secret)
+{
+	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,
 	private_key_exchange_t *this)