_fix_manifest warns about unit values on empty manifest

[kentfredric]

When MANIFEST is a completely empty file, _fix_manifest assumes it has lines and spews warnings:

Use of uninitialized value $manifest[-1] in string eq at /home/kent/perl/forks/ExtUtils-Manifest/lib/ExtUtils/Manifest.pm line 724.
        ExtUtils::Manifest::_fix_manifest("MANIFEST") called at /home/kent/perl/forks/ExtUtils-Manifest/lib/ExtUtils/Manifest.pm line 690
        ExtUtils::Manifest::maniadd(HASH(0x2c2ea80)) called at t/01-filenames.t line 23

Repro:

   path("MANIFEST")->spew_raw("");
   maniadd({ anything => "anyvalue" });