PerfdataTest: Update case which explicitly checks thresholds for null

It's not important whether null or not, as long as it properly detects
that no threshold is set. Further tests specifically for ThresholdRange
should accommodate this case by verifying we don't detect any WARNING.

refs #1298

modules/monitoring/test/php/library/Monitoring/Plugin/PerfdataTest.php

@@ -189,12 +189,12 @@ class PerfdataTest extends BaseTestCase
     /**
      * @depends testWhetherFromStringParsesAGivenStringCorrectly
      */
-    public function testWhetherMissingValuesAreReturnedAsNull()
+    public function testWhetherMissingValuesAreProperlyHandled()
     {
         $perfdata = Perfdata::fromString('test=1;;3;5');
-        $this->assertNull(
-            $perfdata->getWarningThreshold(),
-            'Perfdata objects do not return null for missing warning tresholds'
+        $this->assertEmpty(
+            (string) $perfdata->getWarningThreshold(),
+            'Perfdata objects do not correctly identify omitted warning tresholds'
         );
         $this->assertNull(
             $perfdata->getMaximumValue(),