tests/kvargs: fix to consider empty elements as valid

Empty elements passed to the kvargs parser are silently
ignored. Examples of valid strings:
  - ""
  - ","
  - ",,,,,,key=val,,,,"

Fix the unit tests to conform to this behavior.

Note: the test_invalid_kvargs() function is broken, which explain
why the tests were not failing. It is fixed in the next commit.

Fixes: e495f5435524 ("kvargs: add test case in app/test")
Cc: stable@dpdk.org
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>