[ticket/11546] Fix is_absolute() throws E_NOTICE for empty string

PHPBB3-11546
Merge remote-tracking branch 'nickvergessen/ticket/11546' into develop-olympus

* nickvergessen/ticket/11546:
  [ticket/11546] Fix is_absolute() throws E_NOTICE for empty string
Merge branch 'develop-olympus' into develop

* develop-olympus:
  [ticket/11546] Fix is_absolute() throws E_NOTICE for empty string