Fix ArticleType's attachmentUrl and CreateOrUpdateArticleReplyFeedback's articleReplyUserId

[MrOrz]

Bugfix 1: ArticleType's attachmentUrl field throws error when attachmentHash is empty

The bug was introduced in #284 .

Querying ArticleType's attachmentUrl will trigger Cannot read properties of undefined (reading 'split') error.

The root cause is that for non-image articles, attachmentHash is null. However, we send null to MediaManager to generate URL for attachmentUrl field.

This PR:

• Returns null for attachmentUrl if attachmentHash is null
• Introduces a test case that will fail if this PR is not implemented

Bugfix 2: CreateOrUpdateArticleReplyFeedback records wrong articleReplyUserId on feedback doc

In the image below, the article reply feedback for article 2889dua0k41ro and reply sKR1hn0BnX5-aOa4S29I has its articleReplyUserId being AWA_PbObyCdS-nWhukq-; however, according to the article data, the article reply is actually created by user AVqVwjqQyrDaTqlmmp_a.

This PR

• Fixes the typo in the logic that wrongly grabs the first article reply's author as articleReplyUserId
• Adjust unit test fixture to reproduce the bug and proof that it's fixed

This PR is deployed to staging on 6/25.

[coveralls]

Coverage increased (+0.01%) to 87.544% when pulling b2c3854 on fix-url-resolver into 6540c6c on master.