Skip status and kind filters when all given values are empty

[vipulpandey21]

What this PR does:

In APIService.ListDeployments, the status and kind filters are added only when at least one usable value was collected, instead of reading statuses[0] / kinds[0] on a slice that can be empty.

+		// Empty values are skipped above, so the request may contain no usable
+		// status at all. In that case no status filter should be added.
+		if len(statuses) > 0 {
 			filters = append(filters, datastore.ListFilter{
 				Field:    "Status",
 				Operator: datastore.OperatorEqual,
 				Value:    statuses[0],
 			})
+		}

Same guard for kinds. Behaviour is unchanged when a valid value is given.

Why we need it:

The loop right above already skips empty values, so a request like statuses: [""] leaves the slice empty and the index access panics with index out of range [0] with length 0. The API server has no recovery interceptor, so the panic stops the whole control plane process, not just that request. It is reachable with pipectl deployment list --status=,. Full details and how I checked it are in #7098.

I only added the two guards. I did not change how empty values are treated, since both the handler and pipectl already skip them on purpose.

How I tested it

Added TestListDeploymentsFilters, which checks which filters actually reach the datastore.

Without the fix it reproduces the panic at the old line:

--- FAIL: TestListDeploymentsFilters/ok:_statuses_holding_only_empty_values_adds_no_status_filter
panic: runtime error: index out of range [0] with length 0
    .../pkg/app/server/grpcapi/api.go:553

With the fix:

--- PASS: TestListDeploymentsFilters
    --- PASS: .../ok:_no_optional_filter_is_given
    --- PASS: .../ok:_a_valid_status_is_used_as_a_filter
    --- PASS: .../ok:_statuses_holding_only_empty_values_adds_no_status_filter
    --- PASS: .../ok:_kinds_holding_only_empty_values_adds_no_kind_filter
    --- PASS: .../invalid:_unknown_deployment_status
ok  	github.com/pipe-cd/pipecd/pkg/app/server/grpcapi

go test ./pkg/app/server/grpcapi/... passes, and go build ./cmd/pipecd ./cmd/pipectl is fine.

go vet on that package still prints two copies lock value warnings for RegisterEvent. Those are already on master without my change, so I left them alone.

Which issue(s) this PR fixes:

Fixes #7098

Does this PR introduce a user-facing change?:

• How are users affected by this change: A request whose status or kind values are all empty is handled like no filter was given, instead of crashing the control plane.
• Is this breaking change: No
• How to migrate (if breaking change): Not applicable

[github-actions]

👋 Hi @vipulpandey21, welcome to PipeCD and thanks for opening your first pull request!

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