DocumentsUI: Fix FC AssertionError

Rootcause: In the download app, when the user long press
to select one of the files or pictures, and then click on
the selected file and the delete button at the same time,
it causes the selected to be empty, and causes
selected.isEmpty`value is true, and causes the assertion
is wrong, assertionError occureed.

Use the if statement to circumvent this assertion.

Change-Id: I613c63016ac38a110c81a0c5b96812a0a4d4f75b
CRs-Fixed: 1068713