Condition IF In Power Query's Advanced Editor

I have a field named field, and I would like to see if it is null, but I get an error in the query, my code is this:

let
Condition= Excel.CurrentWorkbook(){[Name="test_table"]}[Content],
field= Condition{0}[fieldColumn],
query1="select * from students",
if field <> null then query1=query1 & " where id = '"& field &"',
exec= Oracle.Database("TESTING",[Query=query1])
in
exec

but I get an error in the condition, do you identify the mistake? I got Expression.SyntaxError: Token Identifier expected.