BUG/MINOR: config: Stopped parsing upon unmatched environment variables

BUG/MINOR: config: Stopped parsing upon unmatched environment variables

When an environment variable could not be matched by getenv(), the
current character to be parsed by parse_line() from <in> variable
is the trailing double quotes. If nothing is done in such a case,
this character is skipped by parse_line(), then the following spaces
are parsed as an empty argument.

To fix this, skip the double quotes character and the following spaces
to make <in> variable point to the next argument to be parsed.

Thank you to @sigint2 for having reported this issue in GH #2367.

Must be backported as far as 2.4.