fix: change blockchain_key to optional

[guardrails[bot]]

:warning: We detected 5 security issues in this pull request:

Vulnerable Libraries (3)

Severity | Details ----- | -------- High | [addressable@2.7.0](https://github.com/openware/peatio/blob/77b2570049c83b394b496f52800e1a7b1d62934a/Gemfile.lock#L55) - **no patch available** High | [nokogiri@2.7.0](https://github.com/openware/peatio/blob/77b2570049c83b394b496f52800e1a7b1d62934a/Gemfile.lock#L236) - **no patch available** High | [puma@2.7.0](https://github.com/openware/peatio/blob/77b2570049c83b394b496f52800e1a7b1d62934a/Gemfile.lock#L334) - **no patch available** More info on how to fix Vulnerable Libraries in [Ruby](https://docs.guardrails.io/docs/en/vulnerabilities/ruby/using_vulnerable_libraries.html?utm_source=ghpr). ---

Insecure Use of Language/Framework API (2)

Docs | Details ----- | -------- [:bulb:](https://docs.guardrails.io/docs/en/vulnerabilities/ruby/insecure_use_of_language_framework_api.html?utm_source=ghpr) | Title: **User Controlled Method Invocation**, Severity: Medium
https://github.com/openware/peatio/blob/77b2570049c83b394b496f52800e1a7b1d62934a/lib/peatio/vault/encrypted_model.rb#L378 [:bulb:](https://docs.guardrails.io/docs/en/vulnerabilities/ruby/insecure_use_of_language_framework_api.html?utm_source=ghpr) | Title: **User Controlled Method Invocation**, Severity: Medium
https://github.com/openware/peatio/blob/77b2570049c83b394b496f52800e1a7b1d62934a/lib/peatio/vault/rails.rb#L43 More info on how to fix Insecure Use of Language/Framework API in [Ruby](https://docs.guardrails.io/docs/en/vulnerabilities/ruby/insecure_use_of_language_framework_api.html?utm_source=ghpr).

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[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
3 Code Smells

No Coverage information
0.0% Duplication

[TheHolyRoger]

Lol.