skip draw when none is detected

ppl_detector.py

@@ -23,7 +23,7 @@ def detect(img):
         return None
     
     print("processing img...")
-    
+
     objects = detector(source)
     persons = [
         o for o in objects 
@@ -32,7 +32,9 @@ def detect(img):
     n = len(persons)
     result = f"it's got {n} {'ppl' if n > 1 else 'person'} in the image"
 
-    draw_bbox(bboxes)
+    if n > 0:
+        draw_bbox(bboxes)
+    
     return source
 
 def set_result(*args):