Do not validate .model.json files with no content/whitespace only (#420)

* Ignore empty/whitespace-only model.json files

* Ignore no return value from model.json files during snapshot

* Use str::from_utf8 instead of String::from_utf8

* Revert "Ignore no return value from model.json files during snapshot"

This reverts commit 0aef16e30a.

* Add test for empty .model.json files

* Change empty .model.json check method

Co-authored-by: Lucien Greathouse <me@lpghatguy.com>

* Format code with cargo fmt

* Use raw string instead

Co-authored-by: Lucien Greathouse <me@lpghatguy.com>

rojo-test/serve-tests/empty_json_model/default.project.json

@@ -0,0 +1,6 @@
+{
+  "name": "empty_folder",
+  "tree": {
+    "$path": "src"
+  }
+}