Make the pixel access token optional and skip pixels without one

[loevgaard]

Fixes #21

Stacked on #40.

Problem

Three places disagreed about whether a pixel needs an access token:

• Configuration marked access_token as isRequired()->cannotBeEmpty(), so a client-side-only setup had to invent a dummy token, even though the README says the token is for server side and the SDK's Pixel::$accessToken is nullable for exactly that reason.
• ConfigurationBasedPixelProvider carried a comment claiming it filters pixels whose id or access token is empty, while the code only filtered empty ids. An unset META_ACCESS_TOKEN therefore produced a pixel with a null token.
• SendEventHandler posted for every pixel regardless. With a null token the field is dropped from the body, Meta answers 400, the SDK throws, and Messenger retries until the message reaches the failure transport. Synchronously it took the page down instead.

Change

• access_token is optional (defaultNull()); id stays required.
• The misleading comment in the provider is replaced with one describing what actually happens.
• SendEventHandler filters out pixels without an access token, logging one warning per pixel with the pixel id, event name and event id, and returns without calling the client when nothing is left. Client side tracking is untouched, since rendering fbq() only needs the id.
• The handler clones the event before narrowing the pixel list, so a synchronously handled command does not mutate the event the application still holds.

Tests

Four unit tests for the handler (sends, skips token-less pixels, sends nothing when none qualify, does not mutate the caller's event) and an extension test asserting pixels: [{id: '1234'}] is accepted.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.07%. Comparing base (1b06004) to head (1d50dd3).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #41      +/-   ##
============================================
- Coverage     67.99%   67.07%   -0.92%     
- Complexity      134      135       +1     
============================================
  Files            30       30              
  Lines           403      413      +10     
============================================
+ Hits            274      277       +3     
- Misses          129      136       +7     

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