feat: Add code coverage for: Invalid signature case and No signature case in Certificate

[rodolfopietro97]

Greetings again, team. I have created the following pull request aiming to enhance the code coverage of the certificate tests. Specifically, the proposed modifications target two instances:

• The signature for verification of certificate is invalid
• The signature for verification of certificate is not present

These amendments aim to improve the robustness and reliability of our bloom filter tests, ensuring a more comprehensive evaluation of its performance and functionality.

Code coverage BEFORE:

Code coverage AFTER:

[coveralls]

coverage: 91.837% (+0.5%) from 91.293% when pulling 52eadf491759b9ecea05aa1cea7636e84839647d on rodolfopietro97:increase-code-coverage-certificate into b35914b5da547628895c627ab6b3abc72264ffef on vechain:master.