Fix #9877: GS could trigger 'Cost: 0' cost message (#9878)

src/misc_gui.cpp

@@ -565,7 +565,7 @@ void ShowEstimatedCostOrIncome(Money cost, int x, int y)
 }
 
 /**
- * Display animated income or costs on the map.
+ * Display animated income or costs on the map. Does nothing if cost is zero.
  * @param x    World X position of the animation location.
  * @param y    World Y position of the animation location.
  * @param z    World Z position of the animation location.
@@ -573,6 +573,9 @@ void ShowEstimatedCostOrIncome(Money cost, int x, int y)
  */
 void ShowCostOrIncomeAnimation(int x, int y, int z, Money cost)
 {
+	if (cost == 0) {
+		return;
+	}
 	Point pt = RemapCoords(x, y, z);
 	StringID msg = STR_INCOME_FLOAT_COST;