Resolve error when no item exist (iOS)

[koji-1009]

Fixes #391

On iOS, readAll (or deleteAll) will return error code errSecItemNotFound(-25300) if the item does not exist in the keychain. This is due to the error code errSecItemNotFound returned by SecItemCopyMatching (or SecItemDelete). This is expected behavior for iOS processing and is not a bug.

https://developer.apple.com/documentation/security/1542001-security_framework_result_codes/errsecitemnotfound

Therefore, this fix modifies the functions to return nil in cases when errSecItemNotFound is returned.

[tenhobi]

We would love to see merged since we also get errSecItemNotFound instead of null. 👍

[juliansteenbakker]

Looks good, thanks for your contribution!

[koji-1009]

@juliansteenbakker Thanks!