NewRelic exporter fails when no service name is provided

[pmm-sumo]

Describe the bug
User report on gitter indicated that NewRelic exporter fails. The quick investigation suggests this might happen when Service Name is not provided.

Steps to reproduce
Send a span with no service name

What did you expect to see?
No segfault

What did you see instead?
Segfault

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x28 pc=0x2c86ba3]

goroutine 94 [running]:
github.com/open-telemetry/opentelemetry-collector-contrib/exporter/newrelicexporter.exporter.pushTraceData(0xc000498780, 0xc0007e0000, 0x3e8bc00, 0xc0007e7340, 0xc00033fe20, 0x0, 0xc000529688, 0xc42f12)
C:/OpenTelemetryWork/opentelemetry-collector-contrib/exporter/newrelicexporter/newrelic.go:88 +0x423
go.opentelemetry.io/collector/exporter/exporterhelper.(*tracesRequest).export(0xc0004b28e0, 0x3e8bc00, 0xc0007e7340, 0x3e8bc00, 0xc0007e7340, 0xc00055a100)

What version did you use?
Windows, apparently v.0.9.0, though simple change in unit test reproduces this on any platform at master.

What config did you use?

receivers:
  otlp/withendpoint:
    protocols:
      grpc:
        endpoint: 127.0.0.1:55680
exporters:
  logging:
    logLevel: debug
  newrelic:
    apikey: my-API-key
  zipkin:
    endpoint: "http://localhost:9411/api/v2/spans"
service:
  pipelines:
    traces:
      receivers: [otlp/withendpoint]
      exporters: [logging, newrelic, zipkin]

Environment
OS: any OS

Additional context
It is not entirely clear if a default service name (as it happens for e.g. Zipkin exporter) or no service name needs to be exported in such scenario - maintainer input needed

[pmm-sumo]

@MrAlias saw this on Gitter recently and filled a report to aid with fixing :)

[MrAlias]

@MrAlias saw this on Gitter recently and filled a report to aid with fixing :)

Thanks @pmm-sumo !