Simply return an empty WritableBuffer, not a nulled one.

Finally allows loading playlist files on a storage plugin.  Commit
297e2747 attempted to implement this, but failed due to this bug.

Necessary for "final" on gcc 4.6.

Use milliseconds precision to reduce rounding errors.

Use milliseconds precision to reduce rounding errors.

Make sure we return the correct type.  This obsoletes the cast
constructor trick.