Currently compiling iOS Arm64 target with the new build system will break with

note: Using new build system
note: Planning build
note: Constructing build description
...
error: An empty identity is not valid when signing a binary for the product type 'Application'. (in target 'Realm2JSON' from project 'RealmCore')

We can avoid this by forcing the build with the old build system using the flag -UseModernBuildSystem=NO however we should aim to use the new build system https://developer.apple.com/documentation/xcode-release-notes/build-system-release-notes-for-xcode-10 as the legacy one is deprecated and will be removed.