Handle corner-case of current cfg being undefined

This is a follow-up to the shadow-out-of-sync fix, which turned out to be
incomplete.

src/shadow.js

@@ -61,7 +61,7 @@ function processServiceCfg(svc, cfgOld, cfgDelta, onCfgDiff) {
 
 
 function processOldReportedCfg(svc, cfgOld, onCfgDiff) {
-  const diff = shadowDiff(cfgOld, svc.getCurrentCfg());
+  const diff = shadowDiff(cfgOld, svc.getCurrentCfg() || {});
   if (!svc.ephemeraldata && diff !== undefined) {
     console.log(`Reported values for ${svc.key} outdated, updating.`);
     onCfgDiff(diff);

test/shadow.js

@@ -104,6 +104,15 @@ const svc7 = new Service({ // fixed read-back cfg
   getCurrentCfg: () => slowClone({ value: 1234 }),
 });
 
+const svc8 = new Service({ // always missing cfg
+  key: 'svc8',
+  outfile: 'svc8.json',
+  outformat: 'JSON',
+  writeout: (cfg) => storeCfg(svc8, cfg),
+  getCurrentCfg: () => undefined,
+});
+
+
 // Cleanup
 
 function clear()
@@ -124,6 +133,7 @@ function clear()
 const shadow1 =
   new Shadow(mock_comms, THING, { svc1, svc2, svc3, svc4, svc5, svc6 });
 const shadow2 = new Shadow(mock_comms, THING, { svc7 });
+const shadow3 = new Shadow(mock_comms, THING, { svc8 });
 
 // Validate initial processing of services/configs
 shadow1.onFetchStatus('accepted', { state: {} }); // fetch pending after new
@@ -272,3 +282,14 @@ assert.deepEqual(mock_comms.last_update_state, {
   desired: null,
 });
 clear();
+
+// Validate that reported shadow data gets cleared if local data missing
+shadow3.fetch();
+shadow3.onFetchStatus('accepted', { state: { reported: { svc8: { foo: 10 }}}});
+assert.deepEqual(mock_comms.last_update_state, {
+  reported: {
+    svc8: { foo: null }
+  },
+  desired: null,
+});
+clear();