handle missing meter observers

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.

Author: brettmc

src/SDK/Metrics/Meter.php

@@ -81,7 +81,7 @@ public function updateConfigurator(Configurator $configurator): void
         $this->config = $configurator->resolve($this->instrumentationScope);
 
         $startTimestamp = $this->clock->now();
-        foreach ($this->instruments->observers[self::instrumentationScopeId($this->instrumentationScope)] as [$instrument, $stalenessHandler, $r]) {
+        foreach ($this->instruments->observers[self::instrumentationScopeId($this->instrumentationScope)] ?? [] as [$instrument, $stalenessHandler, $r]) {
             if ($this->config->isEnabled() && $r->dormant) {
                 $this->metricFactory->createAsynchronousObserver(
                     $this->registry,