devargs: restore empty devargs

devargs: restore empty devargs

Following commit c07691ae1089, an implicit change has been done in the
devargs API.
This triggers problem in virtual pmds that did not check for parameters
validity as it was implicitely valid.

Fix this by restoring the empty argument as "" and add a note in the api.
Restore associated tests.

Fixes: c07691ae1089 ("devargs: remove limit on parameters length")

Reported-by: Tetsuya Mukawa <mukawa@igel.co.jp>
Signed-off-by: David Marchand <david.marchand@6wind.com>
Tested-by: Tetsuya Mukawa <mukawa@igel.co.jp>