fix(engine): prevent Files.Lines panic on empty file

Files.Lines guards against a nil entry but an empty file inside a
chart is stored as a non-nil zero-length byte slice, so the trailing
newline check indexes s[-1] and panics. The engine recovers the panic
into a render error, making every template/install/upgrade/lint that
references the file fail.

Extend the guard to len(f[path]) == 0 and return an empty slice,
matching the behaviour for missing files.

Fixes #32279

Signed-off-by: Mahesh Sadupalli <mahesh.sadupalli@gmail.com>

pkg/engine/files.go

@@ -154,7 +154,7 @@ func (f files) AsSecrets() string {
 // {{ range .Files.Lines "foo/bar.html" }}
 // {{ . }}{{ end }}
 func (f files) Lines(path string) []string {
-	if f == nil || f[path] == nil {
+	if f == nil || len(f[path]) == 0 {
 		return []string{}
 	}
 	s := string(f[path])

pkg/engine/files_test.go

@@ -30,6 +30,8 @@ var cases = []struct {
 	{"story/author.txt", "Joseph Conrad"},
 	{"multiline/test.txt", "bar\nfoo\n"},
 	{"multiline/test_with_blank_lines.txt", "bar\nfoo\n\n\n"},
+	{"empty/empty.txt", ""},
+	{"empty/newline_only.txt", "\n"},
 }
 
 func getTestFiles() files {
@@ -109,3 +111,31 @@ func TestBlankLines(t *testing.T) {
 	as.Equal("bar", out[0])
 	as.Empty(out[3])
 }
+
+func TestLinesEmptyFile(t *testing.T) {
+	as := assert.New(t)
+
+	f := getTestFiles()
+
+	out := f.Lines("empty/empty.txt")
+	as.Empty(out)
+}
+
+func TestLinesNewlineOnlyFile(t *testing.T) {
+	as := assert.New(t)
+
+	f := getTestFiles()
+
+	out := f.Lines("empty/newline_only.txt")
+	as.Len(out, 1)
+	as.Empty(out[0])
+}
+
+func TestLinesMissingFile(t *testing.T) {
+	as := assert.New(t)
+
+	f := getTestFiles()
+
+	out := f.Lines("nonexistent.txt")
+	as.Empty(out)
+}