add test for zero length variable name
add test for zero length variable name

parsing an environment file should give an error in case a zero-length
variable name (definition w/o a variable name) is encountered.

previously these lines went through unnoticed not informing the user about
a potential configuration error.

Signed-off-by: Tom Klingenberg <ee84cff0eebcde9f24dafc5b46fd9dfd21805438@lastflood.net>

cpuguy83/cli,thaJeztah/cli,docker/cli,cpuguy83/cli,docker/cli,thaJeztah/cli