A major problem is that the value being looked-up from LFForms is not going to be NULL if it it blank, it's going to be an empty string.

Best way I've found to deal with situations like this is creating a Stored Procedure on your SQL database.

Here's an example of a script to create a Stored Procedure in MSSQL v.18:

-- ================================================
-- Template generated from Template Explorer using:
-- Create Procedure (New Menu).SQL
--
-- Use the Specify Values for Template Parameters 
-- command (Ctrl-Shift-M) to fill in the parameter 
-- values below.
--
-- This block of comments will not be included in
-- the definition of the procedure.
-- ================================================
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author:        Matthew Tingey
-- Create date:   12 January 2021
-- Description:   Example of Stored Procedure for Laserfiche Answers
-- =============================================
CREATE PROCEDURE ExampleStoredProcedure 
        -- Add the parameters for the stored procedure here
        @unit varchar(50) = '', 
        @house varchar(50) = '',
        @street varchar(50) = ''
AS
BEGIN
        -- SET NOCOUNT ON added to prevent extra result sets from
        -- interfering with SELECT statements.
        SET NOCOUNT ON;

        -- Insert statements for procedure here
        SELECT prop.propertyID
        FROM property_id_lookup_table AS prop
        WHERE 
        (
          prop.unit = @unit
          OR
          @unit IS NULL
          OR
          @unit = ''
          OR
          prop.unit IS NULL
        )
        AND
        prop.house = @house
        AND
        prop.street = @street
END
GO

This is expecting 3 inputs: unit, house, and street, and returns 1 output: propertyID.  It uses a table named property_id_lookup_table and assumes that the inputs house and street will always match the house and street fields in the table.  It assumes the unit input may be blank or NULL or may match the unit value in the table, or the unit value in the table may be NULL.