Improve visual diff code

 - Simplify a check for an error condition
 - Add a test case to avoid regressing the same error

h/static/scripts/directives/annotation.coffee

@@ -133,8 +133,7 @@ AnnotationController = [
     # hasDiff is set to true is there are any targets with a difference
     # shouldShowDiff is set to true if there are some meaningful differences
     #  - that is, more than just uppercase / lowercase
-    diffFromTargets = (targets) ->
-      return {hasDiff: false, shouldShowDiff: false} unless targets?
+    diffFromTargets = (targets = []) ->
       hasDiff = targets.some (t) ->
         t.diffHTML?
       shouldShowDiff = hasDiff and targets.some (t) ->

tests/js/directives/annotation-test.coffee

@@ -127,6 +127,17 @@ describe 'h.directives.annotation', ->
       controller.render()
       assert.isNull(controller.document)
 
+    describe 'when there are no targets', ->
+      beforeEach ->
+        annotation.target = []
+        controller.render()
+        targets = controller.annotation.target
+
+      it 'sets `hasDiff` to false and `showDiff` to undefined', ->
+        controller.render()
+        assert.isFalse(controller.hasDiff)
+        assert.isUndefined(controller.showDiff)
+
     describe 'when a single target has text identical to what was in the selectors', ->
       it 'sets `showDiff` to undefined and `hasDiff` to false', ->
         controller.render()