tests: pass /dev/null as config for mongod

On certain system, such as Darwin, the default mongod binary is a
wrapper adding a default configuration file. Let's specify that there's
no configuration file to read, otherwise a log file is used instead of
stdout.

Change-Id: I56c72cd61399405fd57d6e3f2b1f53ca6a395835
Fix use the fact that empty sequences are false

For sequences(strings, lists, tuples), in order to consistent with
pep8 recommendations, 'if len(seq)' should be replaced with 'if seq',
and 'if not len(seq)' should be replaced with 'if not seq'.

Change-Id: Ie7b6afbde07bb57a05b36d92a079c5355945b262
Closes-Bug: #1265688
Merge "Fix use the fact that empty sequences are false"
Merge "tests: pass /dev/null as config for mongod"