title: PySpark : Dataframe : Numeric + Null column values resulting in NULL instead of numeric value
body: <p>I am facing a problem in  PySpark Dataframe loaded from a CSV file , where my numeric column do have empty values Like below</p>
  
  <pre><code>+-------------+------------+-----------+-----------+
  |  Player_Name|Test_Matches|ODI_Matches|T20_Matches|
  +-------------+------------+-----------+-----------+
  |   Aaron, V R|           9|          9|           |
  |  Abid Ali, S|          29|          5|           |
  |Adhikari, H R|          21|           |           |
  | Agarkar, A B|          26|        191|          4|
  +-------------+------------+-----------+-----------+
  </code></pre>
  
  <p>Casted  those columns to integer and all those empty become null</p>
  
  <pre><code>df_data_csv_casted = df_data_csv.select(df_data_csv['Country'],df_data_csv['Player_Name'],                                        df_data_csv['Test_Matches'].cast(IntegerType()).alias("Test_Matches"),                                       df_data_csv['ODI_Matches'].cast(IntegerType()).alias("ODI_Matches"),                                         df_data_csv['T20_Matches'].cast(IntegerType()).alias("T20_Matches"))
  
  
  +-------------+------------+-----------+-----------+
  |  Player_Name|Test_Matches|ODI_Matches|T20_Matches|
  +-------------+------------+-----------+-----------+
  |   Aaron, V R|           9|          9|       null|
  |  Abid Ali, S|          29|          5|       null|
  |Adhikari, H R|          21|       null|       null|
  | Agarkar, A B|          26|        191|          4|
  +-------------+------------+-----------+-----------+
  </code></pre>
  
  <p>Then I am taking a total , but if one of them is null , result is also coming as null.  How to solve it ?</p>
  
  <pre><code>df_data_csv_withTotalCol=df_data_csv_casted.withColumn('Total_Matches',(df_data_csv_casted['Test_Matches']+df_data_csv_casted['ODI_Matches']+df_data_csv_casted['T20_Matches']))
  
  +-------------+------------+-----------+-----------+-------------+
  |Player_Name  |Test_Matches|ODI_Matches|T20_Matches|Total_Matches|
  +-------------+------------+-----------+-----------+-------------+
  | Aaron, V R  |           9|          9|       null|         null|
  |Abid Ali, S  |          29|          5|       null|         null|
  |Adhikari, H R|          21|       null|       null|         null|
  |Agarkar, A B |          26|        191|          4|          221|
  +-------------+------------+-----------+-----------+-------------+
  </code></pre>