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]
 	}

services/gitdiff/gitdiff_test.go

@@ -601,6 +601,16 @@ func TestDiffLine_GetCommentSide(t *testing.T) {
 	assert.Equal(t, "proposed", (&DiffLine{Comments: []*issues_model.Comment{{Line: 3}}}).GetCommentSide())
 }
 
+func TestDiffLine_GetLineTypeMarker(t *testing.T) {
+	assert.Equal(t, "", (&DiffLine{Content: ""}).GetLineTypeMarker())
+	assert.Equal(t, "+", (&DiffLine{Content: "+added line"}).GetLineTypeMarker())
+	assert.Equal(t, "-", (&DiffLine{Content: "-deleted line"}).GetLineTypeMarker())
+	assert.Equal(t, " ", (&DiffLine{Content: " unchanged line"}).GetLineTypeMarker())
+	// for a real diff line (including hunk header) from diff output, "Content" should always have a prefix char in [" ", "+", "-"].
+	// for other cases, e.g.: a diff line constructed by our code without real diff output, it is undefined behavior at the moment.
+	assert.Equal(t, "", (&DiffLine{Content: "any-content"}).GetLineTypeMarker())
+}
+
 func TestGetDiffRangeWithWhitespaceBehavior(t *testing.T) {
 	gitRepo, err := git.OpenRepositoryLocal(t.Context(), "../../modules/git/tests/repos/repo5_pulls")
 	require.NoError(t, err)