fix: handle null/undefined as last query value in mysql and postgres patchers

[M-TGH]

Description of changes:

Was beforehand throwing a TypeError when trying to access the constructor property on null or undefined:

connection.query('SELECT a FROM b', null);
// without xray --> resolves query
// with xray    --> throws TypeError: Cannot read property 'constructor' of null  

As far as I know null and undefined are the only data types that will encounter this problem, hence a loose null check should suffice.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[haotianw465]

Thank you for reporting the issue. I believe the postgres package has exactly the same issue. https://github.com/aws/aws-xray-sdk-node/blob/master/packages/postgres/lib/postgres_p.js#L47.

Could you add a test for mysql? If you have extra time would you mind making this change to postgres patcher as well? If not we will merge this and fix postgres using the same code you provide. Please let me know.

[M-TGH]

Wasn't too sure what way to test it as it's a bit of an edge case, but these test cases will fail without the patch.

Also added the patch to the postgres patcher as it did indeed have the same behaviour.

Let me know if there's anything else.

[haotianw465]

Thank you that should be enough. We will prepare a release soon. We really appreciate your contribution.