fix(audience-sample-app): resolve CS8601 nullable warnings

Two warnings flagged by Unity's C# compiler in AudienceSample.cs.

OnSendCustomEvent: the dict was Dictionary<string,object> but `props`
(JsonReader.DeserializeObject result) is nullable. Now we only insert
the "properties" key when non-null. Side-benefit: cleaner JSON output
when no props were entered (key omitted vs serialised as null).

RedactPublishableKey: signature changed from `string? Redact(string?)`
to `string Redact(string)`. The only caller already guards with
!string.IsNullOrEmpty before invoking, so the prior nullable parameter
+ early-return-null path was dead. Non-nullable signature lets the
dict insertion in BuildInitConfigEcho compile cleanly.

Author: ImmutableJeffrey

examples/audience/Assets/SampleApp/Scripts/AudienceSample.cs

@@ -154,7 +154,9 @@ private void OnSendCustomEvent() => RunAndLog("track()", () =>
             var f = CaptureCustomEventForm();
             var props = string.IsNullOrEmpty(f.RawProps) ? null : JsonReader.DeserializeObject(f.RawProps);
             ImmutableAudience.Track(f.Name, props);
-            return Json.Serialize(new Dictionary<string, object> { ["event"] = f.Name, ["properties"] = props }, 2);
+            var echo = new Dictionary<string, object> { ["event"] = f.Name };
+            if (props != null) echo["properties"] = props;
+            return Json.Serialize(echo, 2);
         });
 
         // ---- SDK action handlers: consent ----
@@ -349,12 +351,13 @@ private static Dictionary<string, object> BuildConfigEcho(AudienceConfig config)
         }
 
         // Keeps the pk_imapik-test- / pk_imapik- prefix visible; masks the rest.
-        private static string? RedactPublishableKey(string? key)
+        // Caller must guard against null/empty; signature non-nullable so the
+        // dictionary insertion in BuildInitConfigEcho doesn't trip CS8601.
+        private static string RedactPublishableKey(string key)
         {
-            if (string.IsNullOrEmpty(key)) return key;
             const int PrefixChars = 16;
             const string Mask = "…****";
-            return key!.Length <= PrefixChars ? Mask : key.Substring(0, PrefixChars) + Mask;
+            return key.Length <= PrefixChars ? Mask : key.Substring(0, PrefixChars) + Mask;
         }
 
         // ---- Identity helpers ----