fix(python): Align empty-scope URL rendering with Rust client

[jan-auer]

The Python client rendered empty scopes as an empty string, producing /v1/objects/{usecase}//{key}, while the Rust client and server use the _ sentinel, producing /v1/objects/{usecase}/_/{key}. An object written with one client under an empty scope was not addressable by the other.

This also fixes a bug where falsy scope values like 0 or False were rejected as "empty" because the truthiness check ran before str() conversion.

Fixes FS-435

[linear-code]

FS-435

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.69%. Comparing base (e88b7f8) to head (c92164e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #568      +/-   ##
==========================================
+ Coverage   87.67%   87.69%   +0.01%     
==========================================
  Files          93       93              
  Lines       15113    15116       +3     
==========================================
+ Hits        13251    13256       +5     
+ Misses       1862     1860       -2     

Components	Coverage Δ
Rust Backend	92.45% <ø> (ø)
Rust Client	79.89% <ø> (ø)
Python Client	90.69% <100.00%> (+0.48%)	⬆️

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