fix(ios): harden registerPushToken (empty token unregisters, strict hex)

[aramslegit]

Summary

Two small robustness fixes to iOS registerPushToken (ios/ExpoCrispSdkModule.swift). Follow-up to #33 (mentioned there as standalone fixes).

1. Empty token should unregister

guard !token.isEmpty else { return }   // before: empty input is a no-op

Today, passing an empty string early-returns, so there is no way to unregister a push token through the wrapper (e.g. when a user opts out of notifications and you want Crisp to stop routing pushes to the device). The fix treats an empty/whitespace token as "unregister" and clears the device token via CrispSDK.setDeviceToken(Data()).

2. Strict hex parsing

The previous parser silently skipped invalid bytes and accepted odd-length input:

if let byte = UInt8(token[index..<nextIndex], radix: 16) {
  data.append(byte)   // invalid byte → silently dropped
}

A malformed token would register a truncated/garbage device token with no error surfaced to the caller. The fix throws on odd-length or non-hex input (valid APNs tokens are 64 hex chars), so a bad token fails loudly instead of half-registering.

Testing

Both behaviors run in production in a shipping app (1fifty): the empty-token path is used to clear registration on notification opt-out; the strict parse guards the bridge boundary. Verified on a physical iOS device.

Backward compatibility

• Strict hex parsing: a previously-malformed token now throws instead of silently registering a truncated one. Well-formed tokens are unaffected.
• Empty token: previously a no-op, now clears the device token. If any caller relied on registerPushToken('') doing nothing, that changes — but a no-op on empty was almost certainly unintended (there was no other way to unregister).

Android's registerPushToken has the same if (token.isNotEmpty()) early-return, but I left it out of this PR because the Android Crisp SDK has no obvious public "clear token" call to mirror setDeviceToken(Data()); happy to address it too if you can point me at the right API.