handle null functions key in cpu samples (#8940)

Fixes one of the errors shown in #8866.

Author: jakemac53

packages/devtools_app/lib/src/screens/profiler/cpu_profile_model.dart

@@ -1223,7 +1223,7 @@ class _CpuProfileTimelineTree {
     if (isCodeTree) {
       return _code.function!;
     }
-    final function = samples.functions![index].function;
+    final function = samples.functions?[index].function;
     if (function is vm_service.FuncRef ||
         function is vm_service.NativeFunction) {
       return function;
@@ -1263,7 +1263,7 @@ class _CpuProfileTimelineTree {
           // critical since this is only displayed when VM developer mode is
           // enabled.
           (_function as vm_service.FuncRef?)?.location?.script?.uri
-          : samples.functions![index].resolvedUrl;
+          : samples.functions?[index].resolvedUrl;
 
   int? get sourceLine {
     final function = _function;

packages/devtools_app/test/screens/cpu_profiler/cpu_profile_model_test.dart

@@ -122,6 +122,18 @@ void main() {
       );
     });
 
+    test('converts samples with no functions regression test', () {
+      expect(
+        // False positive for this lint, it is used by the matcher.
+        // ignore: discarded_futures
+        CpuProfileData.generateFromCpuSamples(
+          isolateId: goldenSamplesIsolate,
+          cpuSamples: CpuSamples.parse(goldenCpuSamplesJson)!..functions = null,
+        ),
+        completes,
+      );
+    });
+
     test('to json defaults packageUri to resolvedUrl', () {
       const id = '140357727781376-12';