Fix stack overflow crash when decoding null values in JSON

[kieran-osgood-shopify]

The UnkeyedDecodingContainer.decode([Any].Type) method enters an infinite recursion when encountering JSON null values. None of the type-specific decode attempts (Bool, Double, String, etc.) succeed for null, so isAtEnd never becomes true and the container index never advances, causing a stack overflow.

Fix by checking decodeNil() first in both the unkeyed and keyed container extensions. Also add a terminal else-break in the unkeyed loop to prevent infinite loops for any other unexpected value types.

Additionally, handle null values in KeyedDecodingContainer.decode( [String: Any].Type) so null dictionary values are preserved as NSNull rather than silently dropped.

What changes are you making?

---

Before you merge

Important

• I've added tests to support my implementation
• I have read and agree with the Contribution Guidelines.
• I have read and agree with the Code of Conduct.
• I've updated the README.

Releasing a new version of the kit?

• I have bumped the version number in the podspec file.

Releasing a new major version?

• I have bumped the version number in the README.

---

Tip

See the Contributing documentation for instructions on how to publish a new version of the library.