fix(map): Removed NA text for challenge without time estimat

Closes #16454

common/app/Map/Block.jsx

@@ -61,7 +61,9 @@ export class Block extends PureComponent {
         <span>
         { title }
         </span>
-        <span className={ `${ns}-block-time` }>({ time })</span>
+        {
+          time && <span className={ `${ns}-block-time` }>({ time })</span>
+        }
       </div>
     );
   }

common/models/block.json

@@ -37,7 +37,7 @@
     },
     "time": {
       "type": "string",
-      "required": true
+      "required": false
     }
   },
   "validations": [],

seed/index.js

@@ -46,7 +46,7 @@ Observable.combineLatest(
     const isComingSoon = !!challengeSpec.isComingSoon;
     const fileName = challengeSpec.fileName;
     const helpRoom = challengeSpec.helpRoom || 'Help';
-    const time = challengeSpec.time || 'N/A';
+    const time = challengeSpec.time;
     const isLocked = !!challengeSpec.isLocked;
     const message = challengeSpec.message;
     const required = challengeSpec.required || [];