Replace Empty values with nulls in Spark Dataframe

I have a data frame with n number of columns and I want to replace empty strings in all these columns with nulls.

I tried using

val ReadDf = rawDF.na.replace("columnA", Map( "" -> null));

and

val ReadDf = rawDF.withColumn("columnA", if($"columnA"=="") lit(null) else $"columnA" );

Both of them didn't work.

Any leads would be highly appreciated. Thanks.