Fix "undefined" in challenge Title when shared in Facebook

[luizrrodrigues]

[luizrrodrigues]

@SathyaJayabal Deploying.

Tool to check: https://developers.facebook.com/tools/debug/

Expected:
Challenge with prize:

[$200/$100] - Challenge Name

Challenge without prize:

Challenge Name

[SathyaJayabal]

verified

Note: preview image not displayed for staging link.

[SathyaJayabal]

verified on staging (develop)