fix: encode identity search

[kyle-ssg]

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• I have added information to docs/ if required so people know about the feature!
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Changes

Fixes encoding feature search where the search is null.

How did you test this code?

Viewed the identity page where search is null.

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Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-4691	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-4691	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-4691	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-4691	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-4691	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-4691	Finished ✅	Results ✅