fix: avoid spurious deleted line when diffing empty string

[Yanhu007]

Fixes #22

Problem

Diff("", "hello") produces:

-
+hello

The spurious - line appears because strings.Split("", "\n") returns [""] (a slice containing one empty string), not [].

Fix

Add a splitLines helper that returns nil for empty input, so the diff engine treats an empty string as having zero lines instead of one empty line.

Testing

All existing tests pass. Manually verified:

Diff("", "hello\nworld") → "+hello\n+world"  // no spurious - line
Diff("hello\nworld", "") → "-hello\n-world"  // clean
Diff("", "") → ""                             // empty

[Yanhu007]

Duplicate of #34, closing this one.