fix(proto): preserve empty projection when ser/de MemoryScanExec

[buraksenn]

Which issue does this PR close?

• Closes preserve empty projection when ser/de MemoryScanExec #24085

Rationale for this change

Empty projection is not ser/de correctly in MemoryScanExec

What changes are included in this PR?

Use same approach as FilterExec, HashJoinExec etc for projection encode/decode

Are these changes tested?

Yes, new roundtrip test roundtrip_memory_source_empty_projection, which fails on main.

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.89%. Comparing base (66c3840) to head (6e5213f).
⚠️ Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/proto/src/physical_plan/mod.rs	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24087      +/-   ##
==========================================
+ Coverage   80.86%   80.89%   +0.03%     
==========================================
  Files        1101     1102       +1     
  Lines      375446   376107     +661     
  Branches   375446   376107     +661     
==========================================
+ Hits       303592   304256     +664     
+ Misses      53761    53743      -18     
- Partials    18093    18108      +15     

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