fix(dora): change lead time calculator

[kostas-petrakis]

Summary

Fixes the issue where the project_pr_metrics link to wrong deployment when prev_success_deployment_commit_id is empty.

Does this close any open issues?

Closes #8188

Screenshots

I builded a local image, and re-run the data collection through my existing project, the results are now correct (compare with data from issue):

{
"project_pr_metrics": [
	{
		"id" : "github:GithubPullRequest:4:1806504909",
		"created_at" : "2024-11-20 17:06:50.350",
		"updated_at" : "2024-11-20 17:06:50.350",
		"_raw_data_params" : "",
		"_raw_data_table" : "",
		"_raw_data_id" : 0,
		"_raw_data_remark" : "",
		"project_name" : "*************",
		"first_commit_sha" : "3726ef9fce54e1f95717ab0fbe695403cbf850ff",
		"pr_coding_time" : 1,
		"first_review_id" : "github:GithubPrReview:4:1979264065",
		"pr_pickup_time" : 4,
		"pr_review_time" : 1429,
		"deployment_commit_id" : "github:GithubRun:4:435823930:8567339272:https:\/\/github.com\/****\/****",
		"pr_deploy_time" : 15,
		"pr_cycle_time" : 1449,
		"first_commit_authored_date" : "2024-04-04 09:07:51",
		"pr_created_date" : "2024-04-04 09:08:03",
		"first_comment_date" : "2024-04-04 09:11:44",
		"pr_merged_date" : "2024-04-05 09:00:16",
		"pr_deployed_date" : "2024-04-05 09:14:22"
	}
]}