Update MemoryStore.get signature to match other stores

[keller-mark]

The MemoryStore's .get function has a different function signature compared to other stores such as LocalStore. This PR ensures the default value for the prototype parameter is None for the MemoryStore

https://github.com/keller-mark/zarr-python/blob/7d0b62da45f6d8e5f5d93953d71fe9ef76f79e1c/src/zarr/storage/_local.py#L193

More generally, userland code should be able to assume a generic Store API that has consistent function signatures for important methods such as Store.get. This is an important benefit of the Store abstraction.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.86%. Comparing base (7d0b62d) to head (1d6a4cd).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/zarr/storage/_memory.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3581      +/-   ##
==========================================
- Coverage   61.87%   61.86%   -0.01%     
==========================================
  Files          85       85              
  Lines       10134    10137       +3     
==========================================
+ Hits         6270     6271       +1     
- Misses       3864     3866       +2     

Files with missing lines	Coverage Δ
src/zarr/storage/_memory.py	56.43% <33.33%> (-0.71%)	⬇️

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[d-v-b]

thank you!