fix(engine): make toToml return empty string on error

`toToml` was returning `err.Error()` on marshal failure, inconsistent
with `toYaml` and `toJson` — both of which swallow errors and return
an empty string to avoid embedding raw error text in rendered templates.

Align `toToml` with the same convention. Charts relying on the old
behavior (checking for an error string in the output) should use
`mustToToml` (available since #XXXX) to get an explicit failure instead.

Closes #31430

Signed-off-by: Ilya Kiselev <kis-ilya-a@yandex.ru>

pkg/engine/funcs.go

@@ -168,7 +168,8 @@ func fromYAMLArray(str string) []any {
 }
 
 // toTOML takes an interface, marshals it to toml, and returns a string.
-// On marshal error it returns the error string.
+// On marshal error it returns an empty string (errors are swallowed),
+// consistent with toYAML and toJSON.
 //
 // This is designed to be called from a template. Use mustToToml if you need
 // the template to fail hard on marshal errors.
@@ -177,7 +178,8 @@ func toTOML(v any) string {
 	e := toml.NewEncoder(b)
 	err := e.Encode(v)
 	if err != nil {
-		return err.Error()
+		// Swallow errors inside of a template.
+		return ""
 	}
 	return b.String()
 }

pkg/engine/funcs_test.go

@@ -180,6 +180,10 @@ keyInElement1 = "valueInElement1"`,
 			tpl:    `{{ mustToToml . }}`,
 			expect: "foo = \"bar\"\n", // should succeed and return TOML string
 			vars:   map[string]string{"foo": "bar"},
+		}, {
+			tpl:    `{{ toToml . }}`,
+			expect: "",                       // should return empty string and swallow error (not err.Error())
+			vars:   map[int]string{1: "one"}, // non-string key is invalid in TOML
 		},
 	}