codec2: set input buffer to nullptr when work is done

In C2Work contract, when work is returned via onWorkDone, the input
buffer-pack's buffer vector shall contain nullptrs.

Bug: 77501776
Test: CtsMediaTestCases
Test: C2VDAComponent_test
Change-Id: I6e8de1d7f1ee106012f32142333b7cba9824e9c1
3 files changed