fix(cli): Don't set appVersion to empty string

The read-pkg-up module normalises the data in package.json, which means that if version is missing
it gets set to "" (empty string). This causes issues with source maps not matching notifier app
versions, because a missing appVersion != "".

cli.js

@@ -101,7 +101,9 @@ Promise.resolve()
         readPkgUp(conf.projectRoot || process.cwd())
           .then(arg => {
             const pkg = arg && arg.pkg ? arg.pkg : null;
-            if (pkg) conf.appVersion = pkg.version;
+            // only use pkg.version if it's truthy, because read-pkg-up will
+            // set it to "" (empty string) when it's missing
+            if (pkg && pkg.version) conf.appVersion = pkg.version;
           })
       );
     }