Fix houghcircles.py when no circles found

In the C++ equivalent of this example a check is made whether the vector (here in Python we have a list) actually has any circles in it that is whether the Hough circles function has managed to find any in the given image. This check is missing for the Python example and if no circles are found the application breaks.

samples/python/houghcircles.py

@@ -29,10 +29,12 @@ if __name__ == '__main__':
     cimg = src.copy() # numpy function
 
     circles = cv2.HoughCircles(img, cv2.HOUGH_GRADIENT, 1, 10, np.array([]), 100, 30, 1, 30)
-    a, b, c = circles.shape
-    for i in range(b):
-        cv2.circle(cimg, (circles[0][i][0], circles[0][i][1]), circles[0][i][2], (0, 0, 255), 3, cv2.LINE_AA)
-        cv2.circle(cimg, (circles[0][i][0], circles[0][i][1]), 2, (0, 255, 0), 3, cv2.LINE_AA)  # draw center of circle
+    
+    if circles != None: # Check if circles have been found and only then iterate over these and add them to the image
+        a, b, c = circles.shape
+        for i in range(b):
+            cv2.circle(cimg, (circles[0][i][0], circles[0][i][1]), circles[0][i][2], (0, 0, 255), 3, cv2.LINE_AA)
+            cv2.circle(cimg, (circles[0][i][0], circles[0][i][1]), 2, (0, 255, 0), 3, cv2.LINE_AA)  # draw center of circle
 
     cv2.imshow("source", src)
     cv2.imshow("detected circles", cimg)