only iterate props in REPORT request if they are there

This error can be caused by a request with the following content (as done for example by caldavzap):

```
<?xml version="1.0" encoding="utf-8"?>
<A:expand-property xmlns:A="DAV:">
<A:property name="calendar-proxy-read-for" namespace="http://calendarserver.org/ns/">
<A:property name="email-address-set" namespace="http://calendarserver.org/ns/"/>
<A:property name="displayname" namespace="DAV:"/>
<A:property name="calendar-user-address-set" namespace="urn:ietf:params:xml:ns:caldav"/>
</A:property><A:property name="calendar-proxy-write-for" namespace="http://calendarserver.org/ns/"><A:property name="email-address-set" namespace="http://calendarserver.org/ns/"/>
<A:property name="displayname" namespace="DAV:"/>
<A:property name="calendar-user-address-set" namespace="urn:ietf:params:xml:ns:caldav"/>
</A:property>
</A:expand-property>
```