Fix issue with only geter in computed property witchout seter

Author: armano2

lib/utils/index.js

@@ -349,6 +349,6 @@ module.exports = {
       }
 
       return { key, value }
-    })
+    }).filter(cp => cp.value)
   }
 }

tests/lib/rules/no-async-in-computed-properties.js

@@ -34,11 +34,16 @@ ruleTester.run('no-async-in-computed-properties', rule, {
               } finally {
                 return bar
               }
+            },
+            foo2: {
+              get () {
+                new Promise((resolve, reject) => {})
+              }
             }
           }
         }
       `,
-      parserOptions: { ecmaVersion: 6, sourceType: 'module' }
+      parserOptions: { ecmaVersion: 8, sourceType: 'module' }
     }
   ],