Fix possible null pointer deference (#6184)

Signed-off-by: vaidikcode <[email protected]>
## Which problem is this PR solving?
- Resolves #5981 

## Description of the changes
- This will cause a null pointer exception if trace is appended without
the if block being executed.

## How was this change tested?
- 

## Checklist
- [ ] I have read
https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
- [ ] I have signed all commits
- [ ] I have added unit tests for the new functionality
- [ ] I have run lint and test steps successfully
  - for `jaeger`: `make lint test`
  - for `jaeger-ui`: `yarn lint` and `yarn test`

Co-authored-by: Yuri Shkuro <[email protected]>

plugin/storage/grpc/shared/grpc_client.go

@@ -164,7 +164,7 @@ func (c *GRPCClient) FindTraces(ctx context.Context, query *spanstore.TraceQuery
 		}
 
 		for i, span := range received.Spans {
-			if span.TraceID != traceID {
+			if trace == nil || span.TraceID != traceID {
 				trace = &model.Trace{}
 				traceID = span.TraceID
 				traces = append(traces, trace)