fix: skip fetching commits if last generated commit is empty (#1870)

Fixes #1869

Author: JoeWang1127

internal/librarian/generate_test.go

@@ -1571,14 +1571,52 @@ func TestUpdateChangesSinceLastGeneration(t *testing.T) {
 			},
 		},
 		{
-			name:      "failed to get conventional commits",
+			name:      "empty last generated commit",
 			libraryID: "another-id",
 			libraries: []*config.LibraryState{
 				{
 					ID: "example-d",
 				},
 				{
 					ID: "another-id",
+					APIs: []*config.API{
+						{
+							Path: "api/one/path",
+						},
+						{
+							Path: "api/another/path",
+						},
+					},
+				},
+			},
+			repo: &MockRepository{
+				// Set this error to verify the function under test will not
+				// fetch the commits.
+				GetCommitsForPathsSinceLastGenError: errors.New("simulated error"),
+			},
+			want: &config.LibraryState{
+				ID: "another-id",
+				APIs: []*config.API{
+					{
+						Path: "api/one/path",
+					},
+					{
+						Path: "api/another/path",
+					},
+				},
+				Changes: []*config.Change{},
+			},
+		},
+		{
+			name:      "failed to get conventional commits",
+			libraryID: "another-id",
+			libraries: []*config.LibraryState{
+				{
+					ID: "example-d",
+				},
+				{
+					ID:                  "another-id",
+					LastGeneratedCommit: "fake-sha",
 				},
 			},
 			repo: &MockRepository{

internal/librarian/release_please_lite.go

@@ -16,6 +16,7 @@ package librarian
 
 import (
 	"fmt"
+	"log/slog"
 	"strings"
 
 	"github.com/googleapis/librarian/internal/config"
@@ -41,6 +42,10 @@ func GetConventionalCommitsSinceLastRelease(repo gitrepo.Repository, library *co
 // GetConventionalCommitsSinceLastGeneration returns all conventional commits for
 // all API paths in given library since the last generation.
 func GetConventionalCommitsSinceLastGeneration(repo gitrepo.Repository, library *config.LibraryState) ([]*conventionalcommits.ConventionalCommit, error) {
+	if library.LastGeneratedCommit == "" {
+		slog.Info("the last generation commit is empty, skip fetching conventional commits", "library", library.ID)
+		return make([]*conventionalcommits.ConventionalCommit, 0), nil
+	}
 	apiPaths := make([]string, 0)
 	for _, oneAPI := range library.APIs {
 		apiPaths = append(apiPaths, oneAPI.Path)