fix: in js 0 is falsy, which messes with ternary logic

We have this helper function which converts `bigint` to `number`
or raises an error in the unlikely case that it's out of bounds.
And for typing reasons, we want to keep its signature purely
`function(bigint): number`.

But in a few places, we needed to call that function on a value
which might be undefined. So we used ternary logic, like this:

```js
foo? saveBigIntToNumber(foo): undefined
```

And on its face that does the right thing: both `null` and
`undefined` are falsy in JS, so you get `undefined` out.

What we hadn't considered was that the defined number 0 is _also_
falsy in JS, meaning that those instances could change a defined
value to an undefined one, causing some subtle bugs.

So let's get really explicit about this and do things properly.

Author: coriolinus

crypto-ffi/bindings/js/src/Conversions.ts

@@ -9,3 +9,12 @@ export function safeBigintToNumber(x: bigint): number {
     }
     return new Number(x).valueOf();
 }
+
+export function safeBigIntOrUndefinedToNumberOrUndefined(
+    x: bigint | undefined | null
+): number | undefined {
+    if (x === undefined || x === null) {
+        return undefined;
+    }
+    return safeBigintToNumber(x);
+}

crypto-ffi/bindings/js/src/CoreCryptoE2EI.ts

@@ -1,4 +1,4 @@
-import { safeBigintToNumber } from "./Conversions.js";
+import { safeBigIntOrUndefinedToNumberOrUndefined } from "./Conversions.js";
 import * as CoreCryptoFfiTypes from "./autogenerated/core-crypto-ffi.d.js";
 
 import {
@@ -30,7 +30,7 @@ export interface CRLRegistration {
 export function crlRegistrationFromFfi(r: CrlRegistrationFfi): CRLRegistration {
     return {
         dirty: r.dirty,
-        expiration: r.expiration ? safeBigintToNumber(r.expiration) : undefined,
+        expiration: safeBigIntOrUndefinedToNumberOrUndefined(r.expiration),
     };
 }
 

crypto-ffi/bindings/js/src/CoreCryptoMLS.ts

@@ -1,4 +1,4 @@
-import { safeBigintToNumber } from "./Conversions";
+import { safeBigIntOrUndefinedToNumberOrUndefined } from "./Conversions";
 import {
     BufferedDecryptedMessage as BufferedDecryptedMessageFfi,
     CommitBundle as CommitBundleFfi,
@@ -198,9 +198,7 @@ export function bufferedDecryptedMessageFromFfi(
     return {
         message: m.message,
         isActive: m.isActive,
-        commitDelay: m.commitDelay
-            ? safeBigintToNumber(m.commitDelay)
-            : undefined,
+        commitDelay: safeBigIntOrUndefinedToNumberOrUndefined(m.commitDelay),
         senderClientId: m.senderClientId,
         hasEpochChanged: m.hasEpochChanged,
         identity: m.identity,