fix: Resolve the potential panic error in the Go version example in implementing-deduplication.mdx.

[dkkb]

Summary of change

If we don't check the error is nil, the code will panic if there's no error in it.

2024/07/06 21:10:34 [Recovery] 2024/07/06 - 21:10:34 panic recovered:
runtime error: invalid memory address or nil pointer dereference
/usr/local/go/src/runtime/panic.go:261 (0x100495407)
    panicmem: panic(memoryError)
/usr/local/go/src/runtime/signal_unix.go:881 (0x1004953d4)
    sigpanic: panicmem()
/Users/da/github/xxx/server/supertokens.go:161 (0x100d09604)
    InitSuperToken.thirdPartyAPIOverride.func3.1: if err.Error() == "Cannot sign up as email already exists" {
/Users/da/go/pkg/mod/github.com/supertokens/supertokens-golang@v0.22.0/recipe/thirdparty/api/signinup.go:82 (0x100bc09bb)
    SignInUpAPI: result, err := (*apiImplementation.SignInUpPOST)(provider, input, tenantId, options, userContext)
/Users/da/go/pkg/mod/github.com/supertokens/supertokens-golang@v0.22.0/recipe/thirdparty/recipe.go:147 (0x100bc2357)
    (*Recipe).handleAPIRequest: return api.SignInUpAPI(r.APIImpl, tenantId, options, userContext)
/Users/da/go/pkg/mod/github.com/supertokens/supertokens-golang@v0.22.0/supertokens/supertokens.go:261 (0x100a4241f)
    (*superTokens).middleware.func2: apiErr := finalMatchedRecipe.HandleAPIRequest(*id, tenantId, r, dw, theirHandler.ServeHTTP, path, method, userContext)
/usr/local/go/src/net/http/server.go:2166 (0x10073db07)
    HandlerFunc.ServeHTTP: f(w, r)

Related issues

N/A

Checklist

• [x] Algolia search needs to be updated? (If there is a new sub docs project, then yes)
• [x] Sitemap needs to be updated? (If there is a new sub docs project, then yes)
• [x] Checked for broken links? (Run cd v2 && MODE=production npx docusaurus build)
• [x] Changes required to the demo apps corresponding to the docs?

Remaining TODOs for this PR

N/A

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[rishabhpoddar]

Thanks @dkkb