This commit fixes a NPE when the static-locations array of
`ResourceProperties` has to be expanded as the setter is cleaning the
values of the array and is affected by a non-intuitive behaviour of the
binder.

When the binder needs to set an element of an array and the size of the
array isn't large enough, the binder proceeds as follows:

* An array of the required size is created
* The content of the original array is copied over
* The setter of the property is invoked with the new array
* The setter of the property is invoked and the returned array is
mutated to set the requested value

While one would expect the array to contain the requested value when the
setter is invoked, this is not the case. Also, the array might contain
null values if a value at index 8 should be set and the array has a size
of 3.

All in all, `ResourceProperties#appendSlashIfNecessary` has to account
for `null` and an additional round of cleaning has to happen once
binding has completed.

Closes gh-12360