Fix parentCommit invalid memory address or nil pointer dereference. (go-gitea#33204)

When the parent Commit does not exist on gitea, an error will be
reported when opening the Commit details page: invalid memory address or
nil pointer dereference.

![image](https://github.com/user-attachments/assets/4c2a9802-935f-41e9-b5b9-a4f0d745f709)

![image](https://github.com/user-attachments/assets/7b0bc15e-7f5f-4d58-8d24-fee667a799fa)

(cherry picked from commit b7614e2)

Author: hiifong

modules/git/diff.go

@@ -64,7 +64,10 @@ func GetRepoRawDiffForFile(repo *Repository, startCommit, endCommit string, diff
 		} else if commit.ParentCount() == 0 {
 			cmd.AddArguments("show").AddDynamicArguments(endCommit).AddDashesAndList(files...)
 		} else {
-			c, _ := commit.Parent(0)
+			c, err := commit.Parent(0)
+			if err != nil {
+				return err
+			}
 			cmd.AddArguments("diff", "-M").AddDynamicArguments(c.ID.String(), endCommit).AddDashesAndList(files...)
 		}
 	case RawDiffPatch:
@@ -74,7 +77,10 @@ func GetRepoRawDiffForFile(repo *Repository, startCommit, endCommit string, diff
 		} else if commit.ParentCount() == 0 {
 			cmd.AddArguments("format-patch", "--no-signature", "--stdout", "--root").AddDynamicArguments(endCommit).AddDashesAndList(files...)
 		} else {
-			c, _ := commit.Parent(0)
+			c, err := commit.Parent(0)
+			if err != nil {
+				return err
+			}
 			query := fmt.Sprintf("%s...%s", endCommit, c.ID.String())
 			cmd.AddArguments("format-patch", "--no-signature", "--stdout").AddDynamicArguments(query).AddDashesAndList(files...)
 		}

services/gitdiff/gitdiff.go

@@ -1117,7 +1117,10 @@ func GetDiff(ctx context.Context, gitRepo *git.Repository, opts *DiffOptions, fi
 	} else {
 		actualBeforeCommitID := opts.BeforeCommitID
 		if len(actualBeforeCommitID) == 0 {
-			parentCommit, _ := commit.Parent(0)
+			parentCommit, err := commit.Parent(0)
+			if err != nil {
+				return nil, err
+			}
 			actualBeforeCommitID = parentCommit.ID.String()
 		}
 
@@ -1126,7 +1129,6 @@ func GetDiff(ctx context.Context, gitRepo *git.Repository, opts *DiffOptions, fi
 			AddDynamicArguments(actualBeforeCommitID, opts.AfterCommitID)
 		opts.BeforeCommitID = actualBeforeCommitID
 
-		var err error
 		beforeCommit, err = gitRepo.GetCommit(opts.BeforeCommitID)
 		if err != nil {
 			return nil, err