iOS: Fix IOSSurfaceNoopTest (flutter/engine#55621)
Previously, we were passing `nullptr` to the `IOSSurfaceNoop` constructor:
56d060d501/shell/platform/darwin/ios/ios_surface_noop.mm (L23-L24)
which calls the IOSSurface constructor:
56d060d501/shell/platform/darwin/ios/ios_surface.mm (L55-L58)
Which calls `FML_DCHECK(ios_context_)`, which really doesn't like it when `ios_context_` is null.
I will later follow up with the next question of... "what exactly is going on with our CI here?", but this unbreaks my local dev testing workflow for the moment :)
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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