Fix relabel.Regexp zero value marshalling (#14517)

Signed-off-by: Marco Pracucci <marco@pracucci.com>

model/relabel/relabel.go

@@ -213,6 +213,10 @@ func (re Regexp) IsZero() bool {
 
 // String returns the original string used to compile the regular expression.
 func (re Regexp) String() string {
+	if re.Regexp == nil {
+		return ""
+	}
+
 	str := re.Regexp.String()
 	// Trim the anchor `^(?:` prefix and `)$` suffix.
 	return str[4 : len(str)-2]

model/relabel/relabel_test.go

@@ -900,3 +900,16 @@ action: replace
 		})
 	}
 }
+
+func TestRegexp_ShouldMarshalAndUnmarshalZeroValue(t *testing.T) {
+	var zero Regexp
+
+	marshalled, err := yaml.Marshal(&zero)
+	require.NoError(t, err)
+	require.Equal(t, "null\n", string(marshalled))
+
+	var unmarshalled Regexp
+	err = yaml.Unmarshal(marshalled, &unmarshalled)
+	require.NoError(t, err)
+	require.Nil(t, unmarshalled.Regexp)
+}