use nullptr instead of 0 when init'ing event

src/zm_monitor.cpp

@@ -365,12 +365,12 @@ Monitor::Monitor(
   embed_exif( p_embed_exif ),
   delta_image( width, height, ZM_COLOUR_GRAY8, ZM_SUBPIX_ORDER_NONE ),
   ref_image( width, height, p_camera->Colours(), p_camera->SubpixelOrder() ),
-  purpose( p_purpose ),
+  purpose(p_purpose),
   last_motion_score(0),
-  camera( p_camera ),
-  event(0),
-  n_zones( p_n_zones ),
-  zones( p_zones ),
+  camera(p_camera),
+  event(nullptr),
+  n_zones(p_n_zones),
+  zones(p_zones),
   timestamps( nullptr ),
   images( nullptr ),
   privacy_bitmask( nullptr ),