Fix Printer Properties crash for None option values

[SinghCod3r]

Problem

Printer Properties can crash when a printer option has an IPP no-value default.

pycups converts an IPP no-value attribute to Python None. This value can then reach options.OptionWidget(), where numeric and string option handling assumes the value is convertible to the expected type.

This can result in an uncaught TypeError while opening Printer Properties.

The issue is not specific to job-cancel-after; it is a generic handling problem for None option values returned by CUPS.

Root Cause

The value flow is:

CUPS/IPP no-value
        ↓
pycups
        ↓
Python None
        ↓
printer.attributes
        ↓
printerproperties.add_job_option()
        ↓
options.OptionWidget()
        ↓
TypeError

The existing conversion fallback handles ValueError, but None causes TypeError before that fallback can be used.

Fix

Normalize None to the existing empty-value representation at the beginning of OptionWidget().

This allows existing option handling and fallback behavior to process unset values without changing the behavior of valid option values.

The change is intentionally minimal and does not modify the Printer Properties UI logic or CUPS handling.

Tests

Added regression tests covering:

• A valid numeric CUPS value.
• The lower supported boundary.
• A None value with a numeric range.
• A None value without supported values.
• A None value with supported choices.
• Verification that unset values do not mark the option as changed.

Relevant tests pass, along with Python syntax checks and whitespace validation.

Scope

Only the following files are changed:

• options.py
• test_options.py

No unrelated behavior or code paths are modified.

[tillkamppeter]

Looks good for me.