Fix the KCSAN_ENABLED check when building modules

The KCSAN_ENABLED variable is non-empty when the kernel is being built
with KCSAN. This allows us to disable modules that are known to be
broken.

There was a bug where we would check if it was defined. As this is
always the case the KCSAN_ENABLED variable would be set when building
modules so we would never build such a module. Fix this by checking
if the value is empty before passing it on to the module stage.

This doesn't affect how modules are built as the CFLAGS passed to
modules has the correct check.

Reported by: rstone
Sponsored by: Innovate UK