fix(toml): quote empty keys (#2870)

pkg/yqlib/encoder_toml.go

@@ -73,6 +73,9 @@ func (te *tomlEncoder) CanHandleAliases() bool {
 // in a TOML bare key. TOML bare keys may only contain ASCII letters, ASCII
 // digits, underscores, and dashes.
 func tomlKey(key string) string {
+	if key == "" {
+		return `""`
+	}
 	for _, r := range key {
 		if (r < 'A' || r > 'Z') && (r < 'a' || r > 'z') && (r < '0' || r > '9') && r != '_' && r != '-' {
 			return fmt.Sprintf("%q", key)

pkg/yqlib/toml_test.go

@@ -558,6 +558,38 @@ var tomlScenarios = []formatScenario{
 		scenarioType: "decode",
 	},
 	// Roundtrip scenarios
+	{
+		skipDoc:      true,
+		description:  "Roundtrip: empty root key",
+		input:        "\"\" = \"empty\"\nplain = \"value\"\n",
+		expression:   ".",
+		expected:     "\"\" = \"empty\"\nplain = \"value\"\n",
+		scenarioType: "roundtrip",
+	},
+	{
+		skipDoc:      true,
+		description:  "Roundtrip: empty nested table key",
+		input:        "[outer.\"\"]\nvalue = 1\n",
+		expression:   ".",
+		expected:     "[outer.\"\"]\nvalue = 1\n",
+		scenarioType: "roundtrip",
+	},
+	{
+		skipDoc:      true,
+		description:  "Roundtrip: empty inline table key",
+		input:        "entry = { \"\" = \"value\" }\n",
+		expression:   ".",
+		expected:     "entry = { \"\" = \"value\" }\n",
+		scenarioType: "roundtrip",
+	},
+	{
+		skipDoc:      true,
+		description:  "Roundtrip: empty array of tables key",
+		input:        "[[\"\"]]\nname = \"value\"\n",
+		expression:   ".",
+		expected:     "[[\"\"]]\nname = \"value\"\n",
+		scenarioType: "roundtrip",
+	},
 	{
 		description:  "Roundtrip: inline table attribute",
 		input:        rtInlineTableAttr,