fix: default value when not set

[de-sh]

Fixes #1128.

Description

API for setting hottier fails as request expects the fields used_size and `available_size, which it should not.

How to Verify

1. Start parseable instance
2. Try to create hottier in UI/API

Before: request fails, now it doesn't

---

This PR has:

• been tested to ensure log ingestion and log query works.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added documentation for new or modified features or behaviors.

[coveralls]

Pull Request Test Coverage Report for Build 12931641203

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 12.786%

---

Totals
Change from base Build 12929388721:	0.0%
Covered Lines:	2446
Relevant Lines:	19131

---

💛 - Coveralls

[Koustavd18]

LGTM