[SHA256] Cannot load initial commit page

Problem

GitLab returns 500 error we I try to access the initial commit page.

Example: https://staging.gitlab.com/vyaklushin/my-sha256/-/commit/479c421a108facb7975c0ab51450651be7be2c5bd6940e02e879d363dbdd9aa8

Reproduction steps

1. Visit commits history
2. Click on the first commit

Root cause

We use a hard-coded EMPTY_TREE_ID that doesn't work for SHA256 repositories.

We should use 6ef19b41225c5369f1c104d45d8d85efa9b057b53b14b4b9b939dd74decc5321 instead.