fix(policy): return error when approvers evaluation returns nil

[Chief-Rishab]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 5265931860

• 52 of 70 (74.29%) changed or added relevant lines in 9 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 75.966%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/provider/service.go	0	1	0.0%
api/handler/v1beta1/appeal.go	5	9	55.56%
core/appeal/service.go	31	37	83.78%
plugins/providers/metabase/client.go	3	10	30.0%

Totals
Change from base Build 5263088461:	0.03%
Covered Lines:	8136
Relevant Lines:	10710

---

💛 - Coveralls