Fix panic when calling GetWorkflow (#333)

* Fix panic when calling `GetWorkflow` with no run id when that workflow has never run before

internal/internal_workflow_client.go

@@ -92,7 +92,7 @@ type (
 		// GetID return workflow ID, which will be same as StartWorkflowOptions.ID if provided.
 		GetID() string
 
-		// GetRunID return the first started workflow run ID (please see below)
+		// GetRunID return the first started workflow run ID (please see below) - empty string if no such run
 		GetRunID() string
 
 		// Get will fill the workflow execution result to valuePtr,
@@ -311,11 +311,15 @@ func (wc *WorkflowClient) GetWorkflow(ctx context.Context, workflowID string, ru
 	var runIDCell util.OnceCell
 	if runID == "" {
 		fetcher := func() string {
-			execData, err := wc.DescribeWorkflowExecution(ctx, workflowID, runID)
-			if err != nil {
-				wc.logger.Error("error while fetching workflow execution info", err)
+			execData, _ := wc.DescribeWorkflowExecution(ctx, workflowID, runID)
+			wei := execData.GetWorkflowExecutionInfo()
+			if wei != nil {
+				execution := wei.GetExecution()
+				if execution != nil {
+					return execution.RunId
+				}
 			}
-			return execData.GetWorkflowExecutionInfo().GetExecution().RunId
+			return ""
 		}
 		runIDCell = util.LazyOnceCell(fetcher)
 	} else {

internal/internal_workflow_client_test.go

@@ -32,6 +32,7 @@ import (
 	"time"
 
 	workflowpb "go.temporal.io/api/workflow/v1"
+
 	ilog "go.temporal.io/sdk/internal/log"
 
 	"github.com/golang/mock/gomock"
@@ -952,6 +953,18 @@ func (s *workflowRunSuite) TestGetWorkflowNoRunId() {
 	s.Equal(runID, workflowRunNoRunID.GetRunID())
 }
 
+func (s *workflowRunSuite) TestGetWorkflowNoExtantWorkflowAndNoRunId() {
+	describeResp := &workflowservice.DescribeWorkflowExecutionResponse{
+		WorkflowExecutionInfo: nil}
+	s.workflowServiceClient.EXPECT().DescribeWorkflowExecution(gomock.Any(), gomock.Any(), gomock.Any()).Return(describeResp, nil).Times(1)
+	workflowRunNoRunID := s.workflowClient.GetWorkflow(
+		context.Background(),
+		workflowID,
+		"",
+	)
+	s.Equal("", workflowRunNoRunID.GetRunID())
+}
+
 func getGetWorkflowExecutionHistoryRequest(filterType enumspb.HistoryEventFilterType) *workflowservice.GetWorkflowExecutionHistoryRequest {
 	request := &workflowservice.GetWorkflowExecutionHistoryRequest{
 		Namespace: DefaultNamespace,