Fix panics when extracting or reading an index of a zero-length file

[folbricht]

An index created from an empty file contains no chunks (desync make produces one without complaint), but reading such an index back panicked:

• desync extract crashed with slice bounds out of range [:1] with capacity 0 — SeedSequencer.Next() always emits a segment {first: 0, last: 0} even for an empty chunk list, and the assemble worker then slices Chunks[0:1].
• desync cat and desync mount-index crashed with index out of range [0] in NewIndexReadSeeker, which reads Chunks[0].ID unconditionally.

Reproducer:

$ : > empty.bin
$ desync make -s ./store empty.caibx empty.bin
$ desync extract -s ./store empty.caibx out.bin
panic: runtime error: slice bounds out of range [:1] with capacity 0
$ desync cat -s ./store empty.caibx
panic: runtime error: index out of range [0] with length 0

Fixes:

• AssembleFile() returns right after creating and truncating the output file when the index has no chunks.
• NewIndexReadSeeker() tolerates an empty chunk list; findOffset() just tracks the position for an empty index; Read() returns EOF at or past the end.

Both paths verified with the reproducer above and covered by new tests.