Make "characteristics" and "publishers" optional

The app is functional without either of these

src/config.ts

@@ -24,9 +24,9 @@ import { ConsolePublisher, ConsolePublisherImpl } from './publisher/console'
 import { Characteristics } from './characteristics'
 
 export interface Config {
-  characteristics: Characteristics[]
+  characteristics?: Characteristics[]
   circuits: Circuit[]
-  publishers: Publisher[]
+  publishers?: Publisher[]
 }
 
 export const parseConfig = (configFileContents: string): Config => {
@@ -116,7 +116,7 @@ export const resolveAndValidateConfig = (config: Config): Config => {
   }
 
   // Attach poll function to characteristics sensors
-  for (const c of config.characteristics) {
+  for (const c of config?.characteristics ?? []) {
     switch (c.sensor.type) {
       case CharacteristicsSensorType.Iotawatt:
         c.sensor.pollFunc = getIotawattCharacteristicsSensorData
@@ -129,7 +129,7 @@ export const resolveAndValidateConfig = (config: Config): Config => {
 
   // Create publishers. Ignore any manually defined WebSocket publishers, we only support
   // one right now and it's added separately during application startup.
-  for (const publisher of config.publishers) {
+  for (const publisher of config?.publishers ?? []) {
     switch (publisher.type) {
       case PublisherType.InfluxDB: {
         const influxDbPublisher = publisher as InfluxDBPublisher