Handle null values in custom column

[indhu]

I have two columns which have null values I want to create a new custom column which finds the difference between them producing null values as such. 

I am aware that I can convert the null values of two columns to 0 and proceed further but I wish to keep it as such.

I tried using advanced editor and using if conditions in a custom column but couldn't find a way to handle null values.

Code used in custom column:

if [ColumnA] < [ColumnB] then 0 
else if [ColumnA] = "null" or [ColumnB] = "null" then "null"
else [ColumnA] - [ColumnB]

This is how code looks in advance editor:

#"Added Custom1" = Table.AddColumn(#"Removed Columns1", "Result ", each if [ColumnA] < [ColumnB] then 0 
else if [ColumnA] = "null" or [ColumnB] = "null" then "null"
else [ColumnA] - [ColumnB]),
    #"Result" = #"Added Custom1"{0}[#"Result "]

Error I get:

Expression.Error: We cannot convert the value null to type Logical.
Details:
    Value=
    Type=Type

PS: This logic works fine in a calculated column but I wish to work this custom column

Can somebody please point out in the right direction?

Thanks,

Indhu