Prevent error message when port-data-source.js is missing.

[j-maas]

When running npm run start on the elm-pages v3 starter repo, I got an error message about the port-data-source.js missing:

$ npm run start

> start
> elm-pages dev

No `elm-pages.config.mjs` file found. Using default config.
Failed to start port-data-source watcher Error: Build failed with 2 errors:
error: Could not resolve "./port-data-source"
C:\Users\Johannes\Code\elm-pages-3-alpha-starter\elm-pages\generator\src\dev-server.js:162:65: ERROR: [plugin: example] Cannot read property 'outputs' of undefined
    at failureErrorWithLog (C:\Users\Johannes\Code\elm-pages-3-alpha-starter\node_modules\esbuild\lib\main.js:1599:15)
    at C:\Users\Johannes\Code\elm-pages-3-alpha-starter\node_modules\esbuild\lib\main.js:1245:28
    at processTicksAndRejections (internal/process/task_queues.js:93:5) {
  errors: [
    {
      detail: undefined,
      location: null,
      notes: [],
      pluginName: '',
      text: 'Could not resolve "./port-data-source"'
    },
    {
      pluginName: 'example',
      text: "Cannot read property 'outputs' of undefined",
      location: [Object],
      notes: [Array],
      detail: TypeError: Cannot read property 'outputs' of undefined
          at C:\Users\Johannes\Code\elm-pages-3-alpha-starter\elm-pages\generator\src\dev-server.js:162:66
          at C:\Users\Johannes\Code\elm-pages-3-alpha-starter\node_modules\esbuild\lib\main.js:1036:21
          at runOnEndCallbacks (C:\Users\Johannes\Code\elm-pages-3-alpha-starter\node_modules\esbuild\lib\main.js:1041:11)
          at buildResponseToResult (C:\Users\Johannes\Code\elm-pages-3-alpha-starter\node_modules\esbuild\lib\main.js:1243:7)
          at C:\Users\Johannes\Code\elm-pages-3-alpha-starter\node_modules\esbuild\lib\main.js:1352:14
          at C:\Users\Johannes\Code\elm-pages-3-alpha-starter\node_modules\esbuild\lib\main.js:662:9
          at handleIncomingPacket (C:\Users\Johannes\Code\elm-pages-3-alpha-starter\node_modules\esbuild\lib\main.js:759:9)
          at Socket.readFromStdout (C:\Users\Johannes\Code\elm-pages-3-alpha-starter\node_modules\esbuild\lib\main.js:629:7)
          at Socket.emit (events.js:315:20)
          at addChunk (internal/streams/readable.js:309:12)
    }
  ],
  warnings: []
}
Ran elm make ./.elm-pages/Main.elm in 559ms
Ran elm make .elm-pages/Main.elm in 696ms
elm-pages dev server running at <http://localhost:1234>

Since I believe the file is optional, I propose this PR that checks whether it exists. If does not, nothing happens. If the user calls a port data source, they will get an improved error message, if #282 is merged.