Always return false when null value is passed to bp_is_current_component()

This is an improvement on r7361, where only one condition of the component
check was checked against empty values of `$component`.

See #5158

Props megainfo

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7366 cdf35c40-ae34-48e0-9cc9-0c9da1808c22