Default empty namespace to "default" in API client

[lirenjie95]

Fixes #466

Problem

The API client (api/pkg/apis/v1alpha1/utils/apiclient.go) appends namespace to the query string as-is. When a caller passes an empty namespace, the request goes out with ?namespace= and the server looks objects up under an empty namespace instead of the default one.

Fix

• Add a withDefaultNamespace helper that returns "default" for an empty namespace.
• Use it at every url.QueryEscape(namespace) call site in apiclient.go (30 occurrences).

This matches the convention already used elsewhere in the repo (jobs-manager, stage-manager, target-manager, model/objectmeta.go, etc.), where an empty namespace falls back to "default".

Scope

Only point 1 of the issue is covered by this PR. Point 2 (auditing remaining symphony-api.go callers) and point 3 (filling in missing API client methods) are handled separately.

Test

Added apiclient_test.go: an httptest server records the incoming query — GetInstances with an empty namespace asserts namespace=default, and an explicit namespace passes through unchanged.