fix: unable to train settlers if chieftan is present

It is debatable whether or not to allow this, since chieftan would be of
no use once we have 3 settlers and settle a new village. However, if we
accidentally created a chieftan, we can't train settlers until we send
the chieftan to his death.

I think this move could be a predule to a later game mechanics, where
chieftan could fight (talk, actually) against chieftan, if one is
present in village, so taking a village over isn't so easy.

GameEngine/Technology.php

@@ -294,7 +294,7 @@ class Technology {
                 if (!isset($ownunit['hero'])) {
                     $ownunit['hero'] = 0;
                 }
-                $ownunit['hero'] += $movement['hero'];
+                $ownunit['hero'] += (isset($movement['hero']) ? $movement['hero'] : 0);
             }
         }
         return $ownunit;