fix: set values to None so set_missing_values works

[HenningWendtland]

The bug:
Surfaced on further testing of the changes introduced in PR #10:
set_missing_values() does not auto-fill empty strings (''), just None. The Problem was already present for the "sales_taxes_and_charges" field.

Impact:
If a value in the simple subscription in for example the "customer_address" field get's set and unset it is '' in the DB, not None. So the Address is then empty after invoice creation.

Further Note on the Taxes Site:
The Bug did only hit when a Pricing Rule existed and matched. Otherwise (for example if only a Default 19% Template was present or if Item Taxes applied it did not bite)

[greptile-apps]

Confidence Score: 4/5

The PR appears safe to merge, with a non-blocking gap in regression coverage for the newly corrected defaulting behavior.

The normalization matches the documented semantics of optional fields and no behavioral or security defect was established, but the interaction with Sales Invoice defaulting is not directly tested.

Files Needing Attention: simple_subscription/simple_subscription/doctype/simple_subscription/simple_subscription.py

Prompt To Fix All With AI

### Issue 1
simple_subscription/simple_subscription/doctype/simple_subscription/simple_subscription.py:118-129
**Defaulting Behavior Lacks Coverage**

These assignments now normalize three optional values to `None` and rely on the later `set_missing_values()` call to populate address and tax defaults. No regression test covers invoice creation with blank versus explicitly selected values, so changes to Frappe's defaulting behavior or this method could silently break the fix.

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For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "fix: simplify syntax and add taxes case ..." | Re-trigger Greptile}