handle empty meter observers

[brettmc]

if no observers are registered for a meter's instruments, some tests were emitting php warnings:

• foreach() argument must be of type array|object, null given
• Undefined array key XXX

null coalesce to an empty array to make the test happy.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.84%. Comparing base (7eb3abb) to head (46856d6).
Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1490      +/-   ##
============================================
+ Coverage     70.71%   70.84%   +0.13%     
  Complexity     2761     2761              
============================================
  Files           408      408              
  Lines          8341     8341              
============================================
+ Hits           5898     5909      +11     
+ Misses         2443     2432      -11     

Flag	Coverage Δ
8.1	70.45% <100.00%> (+0.01%)	⬆️
8.2	70.69% <100.00%> (+0.13%)	⬆️
8.3	70.65% <100.00%> (+<0.01%)	⬆️
8.4	70.79% <100.00%> (+0.21%)	⬆️
8.5	70.79% <100.00%> (+0.13%)	⬆️

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Files with missing lines	Coverage Δ
src/SDK/Metrics/Meter.php	69.60% <100.00%> (+2.79%)	⬆️

... and 7 files with indirect coverage changes

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[Nevay]

Same fix needed for the foreach ($this->instruments->writers loop.

$mp = (new MeterProviderBuilder())
    ->addReader(new ExportingReader(new InMemoryExporter()))
    ->build();
$mp->getMeter('test')->createObservableCounter('c')->observe(static fn() => null);
$mp->updateConfigurator(Configurator::meter());