test: fix passwd_sanitize()

test: fix passwd_sanitize()

If any of the variables is empty the output is completely messed up,
because replace("", "FOO") puts "FOO" before every single character.

I don't have my full name configured, and this is what I get:

  USER_FULL_NAME=USER_FULL_NAME=USER_FULL_NAME USER_FULL_NAMEsUSER_FULL_NAMEtUSER_FULL_NAMEdUSER_FULL_NAMEoUSER_FULL_NAMEuUSER_FULL_NAMEtUSER_FULL_NAME USER_FULL_NAME=USER_FULL_NAME=USER_FULL_NAME

Let's check for empty strings before doing any replace.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>