[BUGFIX blueprints] Add parans to the attr decorator when there is no transform type (#6054)

Author: JonForest

packages/-ember-data/blueprints/model/index.js

@@ -142,7 +142,7 @@ function nativeAttr(attr) {
       result = "@hasMany('" + name + "')";
     }
   } else if (type === '') {
-    result = '@attr';
+    result = '@attr()';
   } else {
     result = "@attr('" + type + "')";
   }

packages/-ember-data/node-tests/blueprints/model-test.js

@@ -397,7 +397,7 @@ describe('Acceptance: generate and destroy model blueprints', function() {
             .to.contain("import DS from 'ember-data';")
             .to.contain('const { Model, attr } = DS;')
             .to.contain('export default class FooModel extends Model {')
-            .to.contain('  @attr misc;')
+            .to.contain('  @attr() misc;')
             .to.contain("  @attr('array') skills;")
             .to.contain("  @attr('boolean') isActive;")
             .to.contain("  @attr('date') birthday;")