Now Nailgun reports about successful upload of deployment graph
from directory that can even not contain any 'tasks.yaml' files.
This patch fixes:
* check if at least one 'tasks.yaml' file exists in specified
directory path
* check if at least one 'tasks.yaml' file in specified
directory path is not empty
Change-Id: I5de2474b8fbb46780450ee3873af6189a6cf17cc
Closes-Bug: 1619586
(cherry picked from commit 9dfb28e33c)