edje_cc: fix crash on image data initialization

edje_cc: fix crash on image data initialization

Summary:
Since commit a8538bacce1457a91e28b39a6417eaadb4744b50 a crash occurs when
Images.Set.Image is unused.

To avoid a crash, this patch skips setting image data for unused image sets.

Test Plan:

$edje_cc basic.edc

basic.edc
```
collections {
   set {
      name: "bubble";
      image {
         image: "bubble.png" COMP;
      }
   }
}
```

Reviewers: raster

Subscribers: cedric, #reviewers, #committers, eagleeye

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D12063

Change-Id: Ibcb14df831f0a6eb51d69267b379f6480256969f