♻️ refactor: ineffectual assignment and unused variable

[ken8203]

Summary

Fix ineffectual assignment and remove unused variable.

Ref

Here is the result of golangci-lint of middleware:

middleware/basicauth/basicauth_test.go:82:9: ineffectual assignment to `err` (ineffassign)
		resp, err := app.Test(req)
		      ^
middleware/cache/cache_test.go:32:2: SA4006: this value of `resp` is never used (staticcheck)
	resp, err := app.Test(httptest.NewRequest("GET", "/", nil))
	^
middleware/logger/logger.go:368:4: SA9003: empty branch (staticcheck)
			if _, err := cfg.Output.Write([]byte(err.Error())); err != nil {
			^

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