fix(histogram): create new df

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: janani-gurram

superset/utils/pandas_postprocessing/histogram.py

@@ -49,7 +49,7 @@ def histogram(
         groupby = []
 
     # drop empty values from the target column
-    df.dropna(subset=[column], inplace=True)
+    df = df.dropna(subset=[column])
     if df.empty:
         return df