Make GroupGetInitMessage return optional value

Sources/XXClient/Group/Functions/GroupGetInitMessage.swift

@@ -2,21 +2,16 @@ import Bindings
 import XCTestDynamicOverlay
 
 public struct GroupGetInitMessage {
-  public var run: () -> Data
+  public var run: () -> Data?
 
-  public func callAsFunction() -> Data {
+  public func callAsFunction() -> Data? {
     run()
   }
 }
 
 extension GroupGetInitMessage {
   public static func live(_ bindingsGroup: BindingsGroup) -> GroupGetInitMessage {
-    GroupGetInitMessage {
-      guard let data = bindingsGroup.getInitMessage() else {
-        fatalError("BindingsGroup.getInitMessage returned `nil`")
-      }
-      return data
-    }
+    GroupGetInitMessage(run: bindingsGroup.getInitMessage)
   }
 }