fix: Zarr V2 empty filter serialisation

[LDeakin]

It is no longer possible to create non-conformant Zarr V2 filter metadata by setting filters to Some(Vec![]). Zarr V2 arrays with filters: [] can still be read. zarr-python also shared this bug.

[codecov]

Codecov Report

Attention: Patch coverage is 93.93939% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.19%. Comparing base (4775e84) to head (87d4783).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
zarrs_metadata/src/v2/array.rs	93.93%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #147      +/-   ##
==========================================
+ Coverage   82.14%   82.19%   +0.05%     
==========================================
  Files         173      173              
  Lines       24582    24615      +33     
==========================================
+ Hits        20192    20232      +40     
+ Misses       4390     4383       -7     

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