This MR adds tests for a zero public key and a zero signature for BLS signature.

Signature verification returns false as a public key can't be a point at infinity (zero).

Manually testing the MR

dune exec src/lib_bls12_381_signature/test/main.exe -- --file test_signature.ml -m "zero"

Checklist

Document the interface of any function added or modified (see the coding guidelines)
Document any change to the user interface, including configuration parameters (see node configuration)
Provide automatic testing (see the testing guide).
For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
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