rgw: fix checking index_doc_suffix when getting effective key

rgw: fix checking index_doc_suffix when getting effective key

Currently, if the index_doc_suffix is empty which is leaded
by the IndexDocument field is not configurated or is set to
empty string during enabling bucket website function, the
rgw will crash when accessing the static website through the
S3Website enabled RGW instance.

In actually, we have add the necessary checking in the commit
355f392ad26631f44dac250296e96f421d86fb8f, but double checking
in here is better.

Signed-off-by: Enming Zhang <enming.zhang@umcloud.com>