fix: return error instead of panic on missing policy type in LoadPolicyArray

[zjncs]

Description

persist.LoadPolicyArray panics when the rule has no policy type:

func LoadPolicyArray(rule []string, m model.Model) error {
	key := rule[0]
	sec := key[:1]
	...

Two reachable vectors:

• a policy line with a stray leading comma (e.g. , alice, data1 — an easy hand-editing typo) is tokenized by LoadPolicyLine's csv reader into ["", "alice", "data1"], so key is "" and key[:1] panics with slice bounds out of range [:1] with length 0;
• adapters calling the exported LoadPolicyArray directly (the documented helper for database rows) with an empty slice panic on rule[0].

Either way, NewEnforcer/LoadPolicy crashes the process instead of returning an error, although every other malformed-line case (bad quoting, wrong rule size) already returns an error — see HasPolicyEx's "invalid policy rule size: ..." and commit a98973b improving LoadPolicyArray error handling.

Fix: return a descriptive error when the rule is empty or its policy type token is empty.

Testing

Added TestLoadPolicyArrayEmptyPtype (empty type token and empty rule) and TestLoadPolicyLineLeadingComma (line , alice, data1) in persist/persist_test.go.

• Before the fix: both panic — panic: runtime error: slice bounds out of range [:1] with length 0 at persist/adapter.go:78
• After the fix: both return an error (invalid policy rule: missing policy type)

All existing tests in persist still pass, the full go test ./... suite passes (8 packages), and go vet ./persist/ is clean.

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