bug: scalar function with aggregation cause 500

[yeya24]

Describe the bug

    query_fuzz_test.go:160: case 162 error mismatch.
        instant query: scalar(stddev without (job, series) ({series="0"}))
        err1: <nil>
        err2: server_error: server error: 500

11:47:04 cortex-2: level=warn ts=2023-03-10T19:47:04.909216585Z caller=logging.go:86 traceID=1bbdc4b3e4239fb5 msg="POST /api/prom/api/v1/query (500) 12.043959ms Response: \"unexpected result type on instant query: scalar\\n\" ws: false; Accept-Encoding: gzip; Content-Length: 145; Content-Type: application/x-www-form-urlencoded; User-Agent: Go-http-client/1.1; X-Scope-Orgid: user-1; "

In query frontend, we don't have the scalar type when merging because we assume scalar is not shardable. However, scalar(stddev without (job, series) ({series="0"})) is shardable and the result type is scalar.