Allow exitCode to be null and initialize it to null as well

index.d.ts

@@ -63,7 +63,7 @@ interface IDockerComposePushOptions extends IDockerComposeOptions {
 }
 
 interface IDockerComposeResult {
-    exitCode: number;
+    exitCode: ?number;
     out: string;
     err: string;
 }

index.js

@@ -67,7 +67,7 @@ const execCompose = (command, args, options) => new Promise((resolve, reject) =>
   });
 
   const result = {
-    exitCode: 0,
+    exitCode: null,
     err: '',
     out: ''
   };