Fix session get-or-default

[gatesn]

The comments described this get-or-default, but instead it was a panic

[codspeed-hq]

CodSpeed Performance Report

Merging #5662 will improve performances by 12.72%

_{Comparing ngates/session-default (b65128a) with develop (dcd42d2)}

Summary

⚡ 1 improvement
✅ 1157 untouched
⏩ 617 skipped¹

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	encode_specialized	2.7 ms	2.4 ms	+12.72%

Footnotes

1. 617 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[codecov]

Codecov Report

❌ Patch coverage is 25.92593% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.26%. Comparing base (dcd42d2) to head (b65128a).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
vortex-session/src/lib.rs	25.92%	20 Missing ⚠️

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