(ITS#8249) add search response to back-null

[hyc]

Full_Name: Howard Chu
Version: 2.4
OS: 
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (92.251.255.12)
Submitted by: hyc


Sometimes we need a simple backend like back-null, but which returns at least 1
entry on search requests.