[iOS] Resolve SDK 13 Generation

With XCode 11/SDK 13 you now have to set a blank code signing identity along with disabling code signing in order for the project to generate.

cmake/Toolchains/ios.cmake

@@ -32,6 +32,7 @@ set(IOS ON)
 set(CMAKE_CROSSCOMPILING ON)
 set(CMAKE_MACOSX_BUNDLE YES)
 set(CMAKE_XCODE_ATTRIBUTE_CODE_SIGNING_REQUIRED "NO")
+set(CMAKE_XCODE_ATTRIBUTE_CODE_SIGN_IDENTITY "")
 set(CMAKE_XCODE_ATTRIBUTE_IPHONEOS_DEPLOYMENT_TARGET ${IPHONEOS_DEPLOYMENT_TARGET})
 
 # Setup iOS platform unless specified manually with IOS_PLATFORM