fix: null guard on pathHops prevents crash on ADVERT detail (#538) (#540)

## Summary

Fixes #538 — `null is not an object (evaluating 'pathHops.length')`
crash on ADVERT packet detail.

## Root Cause

`getParsedPath` caches its result as `p._parsedPath`. If another code
path (e.g., object spread, API response) sets `_parsedPath = null`, the
cache check (`!== undefined`) passes and returns `null` — causing
`.length` to crash.

Same pattern exists for `getParsedDecoded`.

## Changes

### `public/packet-helpers.js`
- `getParsedPath`: cached return now uses `|| []` to guard against null
cache
- `getParsedDecoded`: cached return now uses `|| {}` to guard against
null cache

### `public/packets.js`
- `renderDetail()` (line ~1440): defensive `|| []` / `|| {}` on
getParsedPath/getParsedDecoded calls
- `buildFlatRowHtml()` (line ~1103): same defensive guards

### `test-frontend-helpers.js`
- Added test: cached `_parsedPath = null` returns `[]`
- Added test: cached `_parsedDecoded = null` returns `{}`

## Testing

All 428 frontend helper tests pass. All 62 packet filter tests pass.

Co-authored-by: you <you@example.com>

public/packet-helpers.js

@@ -10,7 +10,7 @@
  */
 
 window.getParsedPath = function getParsedPath(p) {
-  if (p._parsedPath !== undefined) return p._parsedPath;
+  if (p._parsedPath !== undefined) return p._parsedPath || [];
   var raw = p.path_json;
   if (typeof raw !== 'string') {
     p._parsedPath = Array.isArray(raw) ? raw : [];
@@ -32,7 +32,7 @@ window.clearParsedCache = function clearParsedCache(p) {
 };
 
 window.getParsedDecoded = function getParsedDecoded(p) {
-  if (p._parsedDecoded !== undefined) return p._parsedDecoded;
+  if (p._parsedDecoded !== undefined) return p._parsedDecoded || {};
   var raw = p.decoded_json;
   if (typeof raw !== 'string') {
     p._parsedDecoded = (raw && typeof raw === 'object') ? raw : {};

public/packets.js

@@ -1100,8 +1100,8 @@
 
   // Build HTML for a single flat (ungrouped) packet row
   function buildFlatRowHtml(p) {
-    const decoded = getParsedDecoded(p);
-    const pathHops = getParsedPath(p);
+    const decoded = getParsedDecoded(p) || {};
+    const pathHops = getParsedPath(p) || [];
     const region = p.observer_id ? (observerMap.get(p.observer_id)?.iata || '') : '';
     const typeName = payloadTypeName(p.payload_type);
     const typeClass = payloadTypeColor(p.payload_type);
@@ -1441,8 +1441,8 @@
     const pkt = data.packet;
     const breakdown = data.breakdown || {};
     const ranges = breakdown.ranges || [];
-    const decoded = getParsedDecoded(pkt);
-    const pathHops = getParsedPath(pkt);
+    const decoded = getParsedDecoded(pkt) || {};
+    const pathHops = getParsedPath(pkt) || [];
 
     // Resolve sender GPS — from packet directly, or from known node in DB
     let senderLat = decoded.lat != null ? decoded.lat : (decoded.latitude || null);

test-frontend-helpers.js

@@ -4183,7 +4183,17 @@ console.log('\n=== app.js: routeTypeName/payloadTypeName edge cases ===');
     assertJsonEqual(getParsedPath(p), []);
   });
 
+  test('getParsedPath: cached null _parsedPath returns empty array (#538)', () => {
+    const p = { path_json: '["a"]', _parsedPath: null };
+    assertJsonEqual(getParsedPath(p), []);
+  });
+
   // --- getParsedDecoded ---
+  test('getParsedDecoded: cached null _parsedDecoded returns empty object (#538)', () => {
+    const p = { decoded_json: '{"x":1}', _parsedDecoded: null };
+    assertJsonEqual(getParsedDecoded(p), {});
+  });
+
   test('getParsedDecoded: valid JSON object', () => {
     const p = { decoded_json: '{"type":"GRP_TXT","text":"hello"}' };
     const result = getParsedDecoded(p);