fix: check for Schnorr null signature (#6226)

# Description

## Problem\*

Resolves #6218 

## Summary\*
return false when signature is null


## Additional Context



## Documentation\*

Check one:
- [X] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [X] I have tested the changes locally.
- [X] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

noir_stdlib/src/schnorr.nr

@@ -41,6 +41,8 @@ pub fn verify_signature_noir<let N: u32>(
         for i in 0..32 {
             is_ok &= result[i] == signature[32 + i];
         }
+    } else {
+        is_ok = false;
     }
     is_ok
 }
@@ -92,3 +94,12 @@ fn calculate_signature_challenge<let N: u32>(
     let result = crate::hash::blake2s(hash_input);
     (r.is_infinite, result)
 }
+
+#[test]
+fn test_zero_signature() {
+    let public_key: EmbeddedCurvePoint = EmbeddedCurvePoint { x: 1, y: 17631683881184975370165255887551781615748388533673675138860, is_infinite: false };
+    let signature: [u8; 64] = [0; 64];
+    let message: [u8; _] = [2; 64]; // every message
+    let verified = verify_signature_noir(public_key, signature, message);
+    assert(!verified);
+}