How guard nil in math function if it drops with bad argument in arithmetic expression

Trying figure out, while generating GQL schema,
why if :float data in source var square_plot exists (say 50),
there is no ERR
but if it is NIL the code generates error
while i converting value so sqft with such code:

may be it seems no validation passes

CODE

field(:square_plot, :float)     // my var is `50.0` in DB

field :square_plot_ft, :float do
  if square_plot > 0 do
    resolve(fn _args, %{source: source} -> {:ok, Float.round(source.square_plot*10.7639, 0)} end)
  end
end

if value NOT NIL the GQL response

{
    "data": {
        "unit": {
            "squareTotalFt": 1615.0,
            "squareTotal": 150.0,
            "squarePlotFt": 538.0,
            "squarePlot": 50.0
        }
    }
}

if value is NIL the GQL response

"Internal server error"

and server ERR

** (exit) an exception was raised:
    ** (ArithmeticError) bad argument in arithmetic expression
        (platform) lib/units_web/schema/object.ex:228: anonymous fn/2 in UnitsWeb.Schema.Unit.__abs
inthe_type__/1