EscalationCondition.php: Set default operator value (#511)

For the `incident_age` column, the `operator` select is disabled and
only offers a single option (`>`). Disabled HTML selects don't submit a
value, so the operator was previously missing from the submitted data
and form could not be submitted. Defaulting the element's value to the
first available option ensures it's always set, even while disabled.

Author: BastianLedererIcinga

application/forms/EventRuleConfigElements/EscalationCondition.php

@@ -135,6 +135,7 @@ protected function assemble(): void
         $this->addElement('select', 'operator', [
             'required' => true,
             'options' => $operators,
+            'value' => array_key_first($operators),
             'disabled' => $selectedColumn === 'incident_age'
         ]);