kunit: debugfs: Use IS_ERR() for alloc_string_stream() error check

The alloc_string_stream() function only returns ERR_PTR(-ENOMEM) on
failure and never returns NULL. Therefore, switching the error check in
the caller from IS_ERR_OR_NULL to IS_ERR improves clarity, indicating
that this function will return an error pointer (not NULL) when an
error occurs. This change avoids any ambiguity regarding the function's
return behavior.

Link: https://lore.kernel.org/lkml/Zy9deU5VK3YR+r9N@visitorckw-System-Product-Name


Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>