Add missing return in DexManager

Harmless issue but which can spam the logs.

Test: mostly manual since the check is buried deep inside and cannot be
verified accurately. I added another test to DexManager to stress that
code path and then checked the logs.

Bug: 36117123

(cherry picked from commit 2dfc1b3e)

Merged-In: I1a878a200f3f726dfaa85f1bed1398acc8dce979
Change-Id: Ib2c034add2e8229b90eb4aaa0067d42d26798670