fix(plugin): ToEinoSchemaParameterInfo nil panic

[mrh997]

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Fixes #799

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
backend/api/model/crossdomain/plugin/openapi.go	0.00%	2 Missing ⚠️

Files with missing lines	Coverage Δ
backend/api/model/crossdomain/plugin/openapi.go	12.04% <0.00%> (ø)

... and 6 files with indirect coverage changes

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