[core] Follow-up on the platform integration callback PR

Addressing review comments that arrived after the PR was merged.

include/mbgl/util/run_loop.hpp

@@ -48,14 +48,12 @@ public:
     void runOnce();
     void stop();
 
-    using PlatformCallback = std::function<void()>;
-
     // Platform integration callback for platforms that do not have full
     // run loop integration or don't want to block at the Mapbox GL Native
     // loop. It will be called from any thread and is up to the platform
     // to, after receiving the callback, call RunLoop::runOnce() from the
     // same thread as the Map object lives.
-    void setPlatformCallback(PlatformCallback&& callback) { platformCallback = std::move(callback); }
+    void setPlatformCallback(std::function<void()> callback) { platformCallback = std::move(callback); }
 
     // So far only needed by the libcurl backend.
     void addWatch(int fd, Event, std::function<void(int, Event)>&& callback);
@@ -104,7 +102,10 @@ private:
             defaultQueue.emplace(std::move(task));
         }
         wake();
-        platformCallback();
+
+        if (platformCallback) {
+            platformCallback();
+        }
     }
 
     void process() {
@@ -127,7 +128,7 @@ private:
         }
     }
 
-    PlatformCallback platformCallback = [] {};
+    std::function<void()> platformCallback;
 
     Queue defaultQueue;
     Queue highPriorityQueue;