Import pandas and handle NaN values in Supertrend

Added pandas import and implemented fillna to handle NaN values in the Supertrend strategy.

user_data/strategies/Supertrend.py

@@ -18,6 +18,7 @@ from freqtrade.strategy import IStrategy, IntParameter
 from pandas import DataFrame
 import talib.abstract as ta
 import numpy as np
+import pandas as pd
 
 class Supertrend(IStrategy):
     # Buy params, Sell params, ROI, Stoploss and Trailing Stop are values generated by 'freqtrade hyperopt --strategy Supertrend --hyperopt-loss ShortTradeDurHyperOptLoss --timerange=20210101- --timeframe=1h --spaces all'
@@ -171,8 +172,6 @@ class Supertrend(IStrategy):
         # 3. final upper / lower bands
         final_ub = np.zeros(length)
         final_lb = np.zeros(length)
-        final_ub[:period] = basic_ub[:period]
-        final_lb[:period] = basic_lb[:period]
     
         for i in range(period, length):
             final_ub[i] = basic_ub[i] if basic_ub[i] < final_ub[i-1] or close[i-1] > final_ub[i-1] else final_ub[i-1]
@@ -189,4 +188,9 @@ class Supertrend(IStrategy):
         # 5. STX direction
         stx = np.where(st > 0, np.where(close < st, 'down', 'up'), None)
     
-        return pd.DataFrame({'ST': st, 'STX': stx}, index=df.index)
+        # 6. fillna
+        result = pd.DataFrame({'ST': st, 'STX': stx}, index=df.index)
+        result.fillna(0, inplace=True)
+      
+        return result
+