[Windows] Fix crash if the Dart entry point is null

[loic-sharma]

The Windows Flutter engine crashes if it is launched with a nullptr Dart entry point:

strlen()
std::_Narrow_char_traits<char,int>::length
std::basic_string_view<char,std::char_traits<char>>::basic_string_view
FlutterDesktopEngineRun
flutter::FlutterEngine::Run(const char*)
flutter::FlutterEngine::Run()
wWinMain

This is needed for scenarios like headless mode with no views. Fixes flutter/flutter#116753

I'll prepare a cherry pick after this is merged.

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• All existing and new tests are passing.

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[yaakovschectman]

lgtm

[cbracken]

Ouch! Thanks for fixing!