shared.c: Only setenv() if value is non-null

Some setenv() implementations (e.g. the one in OpenBSD's stdlib)
segfault if we pass a NULL value. Only set environment variables if the
corresponding settings are defined to avoid this.

Note that this is a minor behaviour change as environment variables were
supposed to be set to an empty string if a setting was undefined. Given
that this feature isn't part of any official release yet, there's no
need to worry about backwards compatibility, really. Change the
documentation accordingly.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>

cgitrc.5.txt

@@ -500,7 +500,7 @@ Also, all filters are handed the following environment variables:
 
 If a setting is not defined for a repository and the corresponding global
 setting is also not defined (if applicable), then the corresponding
-environment variable will be an empty string.
+environment variable will be unset.
 
 
 MACRO EXPANSION