Get error when affect a String Field in a string var

I want to know how to elimate an error when i affect the content of a
field in a TTable in a string variable and the content of the field is
empty.
If the content is non-empty, no problem.
How i can't let a field empty without the error conversion.
I don't want to check all fields who can be empty.
I expect to have a right to let a field blank.

Exemple:

Employee: TTable;   // Paradox table

        EmployeNo       String  4
        EmployeeName    String  30
        EmployeeTel1    String  10
        EmployeeTel2    String  10

procedure EmployeeReadRecord;
var
        lcTel2: string[10];
begin
        lcTel2 := Employee['EmployeeTel2'];
end;

The problem is the same if i use the

        lcTel2 := Employee.FieldByName( 'EmployeeTel2' ).AsString;