RefreshLinksJob: Replace use of empty() with explicit check

RefreshLinksJob: Replace use of empty() with explicit check

Using empty() is a maintenance nightmare as it completely removes
any meaning of what the author intends to check for (whether the
variable exists at all, it being set to an empty array, 0, '0' etc.)

Follows-up I445e10da122e5f24e589 which changed it from isset()
to !empty() with commit message of checking for false.

Change-Id: I561f03b0526685f51138d723f9eb7196725e0b93