fix(media-all): only call media.get if there is an id

[rowasc]

This pull request makes the following changes:

Testing checklist:

• [ ]

• I certify that I ran my checklist

Fixes ushahidi/platform# .

Ping @ushahidi/platform

[Angamanga]

🎉

[Angamanga]

@Obadha2 This fix has no issue so adding the QA-checklist here (is in Steve Buscemi and in production already):

• Add a post to a survey with images, but leave the image-field empty (no image)
• Save the post
• Go to dataview and look at the post
• The post should load without any 404-responses from the /media endpoint (you can check this in the inspector, 'network'-tab and filter on "media":
  
• Select edit for this post
• Still no 404-responses from the /media endpoint
• Add an image
• The image should load
• Save the post
• The image should load when viewing the post

[AmTryingMyBest]

QA'd, passes. 👍