The gogit implementation of git notes differs from the nogogit implementation

[nitul1991]

• Gitea version (or commit ref): 1dc41c4
• Git version: 2.32.0
• Operating system: macOS Catalina version 10.15.6
• I built gitea from source and ran the tests via the command-line.
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
• Log gist:

Description

Whilst working on the issue #15373, I discovered that the implementation of the gogit version of git notes differs from the no-gogit version when a non existent commit ID is passed to the function GitNote.
I brought this up on Discord and @zeripath suggested that I provide a fix for this issue in a separate PR so that it can be backported to version 1.15.