Composer: Not assuming element has leaves

Summary:
Fixes a bug that where two dates are mentioned in a row, the second date
doesn't have leaf elements (not sure what those are) and explodes.
Instead, the second date just won't be highlighted

This affects more than just dates, as one can see in the attached task.

Reviewed By: claudiopro

Differential Revision: D16756786

fbshipit-source-id: 929600e1a5258120c70f2321eeace651afee3670

src/component/contents/DraftEditorBlock.react.js

@@ -147,6 +147,10 @@ class DraftEditorBlock extends React.Component<Props> {
     return this.props.tree
       .map((leafSet, ii) => {
         const leavesForLeafSet = leafSet.get('leaves');
+        // T44088704
+        if (leavesForLeafSet.size === 0) {
+          return null;
+        }
         const lastLeaf = leavesForLeafSet.size - 1;
         const leaves = leavesForLeafSet
           .map((leaf, jj) => {