Fix TargetIDSet.IsEmpty and add regression test

[AleksaMCode]

Description

Fixes an inverted emptiness check in lambda target ID sets and adds a regression test.

• Updated internal/config/lambda/event/targetidset.go:
  • TargetIDSet.IsEmpty() now correctly returns len(set) == 0.
• Added TestTargetIDSetIsEmpty in:
  • internal/config/lambda/event/targetidset_test.go.

Motivation and Context

I discovered this while evaluating Silo as a MinIO replacement for my production CI/CD environment.

TargetIDSet.IsEmpty() was returning true for non-empty sets (len(set) != 0), which is logically incorrect and could cause future call sites to invert control flow. This PR fixes the behavior and adds a dedicated unit test so the bug cannot regress silently.

How to test this PR?

go test ./internal/config/lambda/event

Compatibility impact

No API, protocol, config key, metric, header, route, storage format, or module path changes.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• All commits are signed off (git commit -s) per the DCO
• Fixes a regression (If yes, please add commit-id or PR # here)
• Unit tests added/updated
• make verifiers passes
• Relevant package tests and make build pass
• Compatibility and rollback impact documented
• Internal documentation updated
• Public documentation update opened in pgsty/silo.pgsty.com, if needed