fix(@angular-devkit/core): make workspace projects field not required

It has a default which makes the required aspect void.
Global files also do not have projects.

Author: clydin

packages/angular_devkit/core/src/workspace/workspace-schema.json

@@ -48,8 +48,7 @@
   },
   "additionalProperties": false,
   "required": [
-    "version",
-    "projects"
+    "version"
   ],
   "definitions": {
     "project": {