ok so let us keep it simple:

df1 = spark.read.format("csv").option("header","true").load("file:///home/cloudera/data/a.csv")
 
if "col" in df1.columns:
 df2 = df1
else:
 df2 = df1.withColumn("col", lit(None).cast(TypeYouWant)

Next you can use your case etc on df2.