iOS: Change engine NSAssert to FML_CHECK (#166009)

We assume the engine is non-nil everywhere in the engine, further, this
is a one-time check with near-zero runtime performance -- we're not
creating view controllers at any frequency that will cause a nil check
to cause any impact. This also guarantees that the device logs an error
message that makes clear exactly what happened, even in release builds.

This guarantees that both in debug and release builds, we get useful
logging if a nil engine is passed to the initializer.

The initializer *does* declare the FlutterEngine* parameter to be
non-nil, but this only provides a hint to the compiler and doesn't
prevent nil engines being passed at runtime, or really at compile time;
in our clang toolchain, the following compiles fine:

```objc
FlutterEngine* engine = nil;
FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:engine
                                                                              nibName:nil
                                                                               bundle:nil];
```

Long-term, we should resolve
https://github.com/flutter/flutter/issues/157837 and migrate from
FML_CHECK and FML_DCHECK, to using NSAssert agressively in the embedder.

For further details, see my comments here:
https://github.com/flutter/flutter/issues/153971#issuecomment-2755404075

No test changes since this introduces no changes to the debug-mode
builds used in testing.

Fixes: https://github.com/flutter/flutter/issues/153971

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