glib/tests/enviroment.c: Fix running on Windows
glib/tests/enviroment.c: Fix running on Windows

Don't attempt to insert environmental variables in the hash table during
the test listenv that is an empty string, as GetEnvironmentStringsW() also
returns special enviroment variables which have empty strings as their
variable names, at least on Windows 7 and 8.

https://bugzilla.gnome.org/show_bug.cgi?id=711047

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