HP-2212 Fix falling phpunit-legacy Unit tests after add Since and Till properties to Bill (#91)

* HP-2212 Fix falling phpunit-legacy Unit tests

* HP-2212 initialize default value for Bill::$usageInterval property

Author: VadymHrechukha

src/bill/Bill.php

@@ -89,7 +89,6 @@ public function __construct(
         $this->plan         = $plan;
         $this->charges      = $charges;
         $this->state        = $state;
-        $this->setUsageInterval(UsageInterval::wholeMonth($time));
     }
 
     /**
@@ -111,9 +110,18 @@ public function getUniqueString(): string
 
     public function getUsageInterval(): UsageInterval
     {
+        if ($this->usageInterval === null) {
+            $this->initializeWholeMonthUsageInterval();
+        }
+
         return $this->usageInterval;
     }
 
+    private function initializeWholeMonthUsageInterval(): void
+    {
+        $this->setUsageInterval(UsageInterval::wholeMonth($this->time));
+    }
+
     public function calculatePrice()
     {
         $quantity = $this->quantity->getQuantity();