fix(gitdiff): prevent index out of range panic in GetLineTypeMarker (#38728)

Fixes a `runtime error: index out of range [0] with length 0` panic when
executing template `repo/diff/section_unified` because
`GetLineTypeMarker()` indexes into empty `DiffLine` content.

The regression was introduced in PR #38706 (`94c61137e4`) where
`Content: " "` was removed from the initialization of `tailDiffLine`.
Guarding `GetLineTypeMarker()` directly makes Gitea defensive and robust
against empty diff line values regardless of the source path.

Fixes  https://github.com/go-gitea/gitea/issues/38724

### Tests
- Added `TestDiffLine_GetLineTypeMarker` covering empty content,
prefixes, and normal text formats.

Signed-off-by: Sudhanshu Singh <sudhanshuwriterblc@gmail.com>

services/gitdiff/gitdiff.go

@@ -170,6 +170,9 @@ func (d *DiffLine) GetCommentSide() string {
 
 // GetLineTypeMarker returns the line type marker
 func (d *DiffLine) GetLineTypeMarker() string {
+	if d.Content == "" {
+		return ""
+	}
 	if strings.IndexByte(" +-", d.Content[0]) > -1 {
 		return d.Content[0:1]
 	}