fix: handle empty format spec in __format__

[underwoo]

Fixes #3

format(fsize, "") now returns str(fsize) instead of the K-unit value, satisfying Python's data model requirement that format(x, "") == str(x).

Changes:

• Added early guard clause if not format_spec: return str(self) at the top of FSize.__format__()
• Updated __format__ docstring to document empty-spec behavior
• Added test_format_empty_spec() covering bytes, MiB, and KB constructors

Verification:

Check	Result
pytest (46 tests)	All passed
mypy	No issues
pylint	Clean

Closes #3