UnknownProperty: handle null values

[rfc2822]

ICalendars containing:

ATTACH;ENCODING="BASE64,BASE64";ID=rfc2445.ics;VALUE=BINARY:

seemingly cause the value of the parsed iCalendar property to be null (makes sense, because the value couldn't be decoded, because BASE64,BASE64 is not a valid encoding).

So the property.value.length() access causes an NPE:

ical4android/src/main/java/at/bitfire/ical4android/AndroidEvent.kt

Line 993 in f95d63c

if (property.value.length > UnknownProperty.MAX_UNKNOWN_PROPERTY_SIZE) {

Expected result:

Unknown properties without value should be ignored.