[TM-297] Add more tests

Description

Problem: There is a case that wasn't covered by the tests previously.

Solution: Cover it, also add one more test for the case, when parser fails.

Related issue(s)

https://issues.serokell.io/issue/TM-297

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • README
    • Haddock
    • docs/

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.