fix(crypto): check for nil client secret (#7729)

When creating an app without secret or other type of authentication method,
like JWT, and the authentication type is switched afterwards the app would remain without generated secret.
If then client authentication with secret is attempted, for example on the token endpoint, the handler would panic in the crypto.CompareHash function on the nile pointer to the CryptoValue.

This fix introduces a nil pointer check in crypt.CompareHash and returns a error.

The issue was reported over discord: https://discord.com/channels/927474939156643850/1222971118730875020
Possible fix was suggested here: https://github.com/zitadel/zitadel/pull/6999#discussion_r1553503088
This bug only applies to zitadel versions <=2.49.1.

internal/static/i18n/zh.yaml

@@ -314,6 +314,7 @@ Errors:
       APIAuthMethodNoSecret: 选择的 API 身份验证方法不需要秘钥
       AuthMethodNoPrivateKeyJWT: 选择的身份验证方法不需要 Key
       ClientSecretInvalid: Client Secret 无效
+      ClientSecretNotSet: 未设置客户端密码
       Key:
         AlreadyExisting: 已经存在的应用钥匙
         NotFound: 未找到应用钥匙