Add `Aggregate()` case

tests/formats/bls/aggregate.md

@@ -8,11 +8,11 @@ The test data is declared in a `data.yaml` file:
 
 ```yaml
 input: List[BLS Signature] -- list of input BLS signatures
-output: BLS Signature -- expected output, single BLS signature
+output: BLS Signature -- expected output, single BLS signature or empty.
 ```
 
-`BLS Signature` here is encoded as a string: hexadecimal encoding of 96 bytes (192 nibbles), prefixed with `0x`.
+- `BLS Signature` here is encoded as a string: hexadecimal encoding of 96 bytes (192 nibbles), prefixed with `0x`.
-
+- No output value if the input is invalid.
 
 ## Condition
 

tests/generators/bls/main.py

@@ -127,6 +127,19 @@ def case03_aggregate():
             'output': encode_hex(bls.Aggregate(sigs)),
         }
 
+    # Invalid pubkeys -- len(pubkeys) == 0
+    try:
+        bls.Aggregate([])
+    except Exception:
+        pass
+    else:
+        raise Exception("Should have been INVALID")
+
+    yield f'aggregate_na_pubkeys', {
+        'input': [],
+        'output': None,
+    }
+
 
 def case04_fast_aggregate_verify():
     for i, message in enumerate(MESSAGES):