[Bug] if helper returns null instead of undefined

[Turbo87]

🐞 Describe the Bug

The else path of an inline if helper used to return undefined, but starting with v3.25 it now returns null, which can be problematic in components that check for the existence of an argument via this.args.foo !== undefined.

🔬 Minimal Reproduction

<SomeComponent @foo={{if this.bar "baz"}} />

😕 Actual Behavior

When bar is truthy then foo will be "bar", if not, then it will be null.

🤔 Expected Behavior

When bar is truthy then foo will be "bar", if not, then it will be undefined.

🌍 Environment

• Ember: 3.25.1

[rwjblue]

This is a bit unfortunate to care about 🤔. We definitely can fix it, but I'm not sure that this is something that we had explicitly intended. 🤔

[rwjblue]

The inline if implementation is here:

https://github.com/glimmerjs/glimmer-vm/blob/e4be62c16360fba63388a4316a6562a57954bbb4/packages/%40glimmer/runtime/lib/compiled/opcodes/expressions.ts#L245-L259

But the thing that returns null I think is done at compilation time here:

https://github.com/glimmerjs/glimmer-vm/blob/e4be62c16360fba63388a4316a6562a57954bbb4/packages/%40glimmer/compiler/lib/passes/2-encoding/expressions.ts#L153

[rwjblue]

Oh, also, you can work around this by explicitly returning undefined in the else:

<SomeComponent @foo={{if this.bar "baz" undefined}} />

[Turbo87]

This is a bit unfortunate to care about

maybe, but it looks like we're not the only ones who got bitten by this: https://discord.com/channels/480462759797063690/480462759797063692/809912104240545812

in our case about 900 tests are failing in the test suite due to this unintentional change 😅

I guess usually I would just do if (this.arg.foo) {...}, but since @foo can be a number in our case and that number is allowed to be zero we went for the this.arg.foo !== undefined check.