SoftAVCDec: Handle zero length input without EOS

When a zero length input buffer is sent to SoftAVC decoder
without EOS set in nFlags, SoftAVC decoder plugin was
entering an infinite loop. Fixed it by returning from
onQueueFilled for such cases.

Bug: 66969349
Test: Tested using poc associated with the bug
Change-Id: I79cebc8f5b40c51256aba83a08deb547b220c4d7
(cherry picked from commit 21943c05)