Set default search terms before function params are parsed in bp_has_blogs()

By getting the default value of the search_terms parameter from $_REQUEST
before parameters are parsed, we ensure that 'search_terms' is passed to the
bp_parse_args() filter in the same way as the rest of the function params.

See #5456 and r8116 for a similar change in bp_has_groups()

See #5660

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8447 cdf35c40-ae34-48e0-9cc9-0c9da1808c22