Adds support to securely fetch elements from hashtables with default values

lib/core/tools/Get-IcingaHashtableItem.psm1

@@ -0,0 +1,18 @@
+function Get-IcingaHashtableItem()
+{
+    param(
+        $Hashtable,
+        $Key,
+        $NullValue = $null
+    );
+
+    if ($null -eq $Hashtable) {
+        return $NullValue;
+    }
+
+    if ($Hashtable.ContainsKey($Key) -eq $FALSE) {
+        return $NullValue;
+    }
+
+    return $Hashtable[$Key];
+}