AggregationFuzzer: Remove functions with nullptr in custom verifier.

[kgpai]

Description

Currently in the AggregationFuzzer if we have customVerification enabled but the verifier is a nullptr, then we do not do not do any verification of the functions , see : https://github.com/facebookincubator/velox/blob/main/velox/functions/prestosql/fuzzer/AggregationFuzzerTest.cpp#L164 . In these cases , we should have these functions in the skiplist or create a Noop verifier so that we are explicit that running these functions in the Aggregate fuzzer only checks for completion of the aggregate function (or validates that exceptions are thrown by all plans).

[kgpai]

cc: @kevinwilfong

[mbasmanova]

The original behavior was to verify results of order-sensitive functions using aggregations over sorted inputs. We now have order-sensitive flag in the registry, hence, can enhance the Fuzzer to check that flag.