fix: update the precision parameter default value

In influxdata#10803, precision parsing was made more strict so that incorrect
values would be caught. The default precision value however is not
correct as an empty string, but will still accept the emptry string.

fixes: influxdata#10813

config/config.go

@@ -381,7 +381,7 @@ var agentConfig = `
   ##
   ## Precision will NOT be used for service inputs. It is up to each individual
   ## service input to set the timestamp at the appropriate precision.
-  precision = ""
+  precision = "0s"
 
   ## Log at debug level.
   # debug = false

config/types.go

@@ -38,6 +38,10 @@ func (d *Duration) UnmarshalTOML(b []byte) error {
 	// Finally, try value is a TOML string (e.g. "3s", 3s) or literal (e.g. '3s')
 	durStr = strings.ReplaceAll(durStr, "'", "")
 	durStr = strings.ReplaceAll(durStr, "\"", "")
+	if durStr == "" {
+		durStr = "0s"
+	}
+
 	dur, err := time.ParseDuration(durStr)
 	if err != nil {
 		return err

config/types_test.go

@@ -52,6 +52,14 @@ func TestDuration(t *testing.T) {
 	require.NoError(t, d.UnmarshalTOML([]byte(`1.5`)))
 	require.Equal(t, time.Second, time.Duration(d))
 
+	d = config.Duration(0)
+	require.NoError(t, d.UnmarshalTOML([]byte(``)))
+	require.Equal(t, 0*time.Second, time.Duration(d))
+
+	d = config.Duration(0)
+	require.NoError(t, d.UnmarshalTOML([]byte(`""`)))
+	require.Equal(t, 0*time.Second, time.Duration(d))
+
 	require.Error(t, d.UnmarshalTOML([]byte(`"1"`)))  // string missing unit
 	require.Error(t, d.UnmarshalTOML([]byte(`'2'`)))  // string missing unit
 	require.Error(t, d.UnmarshalTOML([]byte(`'ns'`))) // string missing time