Core: Page title: make the 2nd/3rd params for `bp_modify_page_title` optional and provide defaults.

Prevents PHP Warnings caused by other plugins/themes.

Fixes #5933, props imath and whyisjake.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9108 cdf35c40-ae34-48e0-9cc9-0c9da1808c22