Fix IStructureElementChain returning an invalid Iterable

[Lyfts]

You shouldn't concat null iterables as that will just cause the following whenever you try to iterate over it.

[STDERR/]: [java.lang.Throwable$WrappedPrintStream:println:787]: java.lang.NullPointerException: Cannot invoke "java.lang.Iterable.iterator()" because "from" is null
[STDERR/]: [java.lang.Throwable$WrappedPrintStream:println:787]: 	at System//com.google.common.collect.Iterables$3.transform(Iterables.java:512)
[STDERR/]: [java.lang.Throwable$WrappedPrintStream:println:787]: 	at System//com.google.common.collect.Iterables$3.transform(Iterables.java:509)
[STDERR/]: [java.lang.Throwable$WrappedPrintStream:println:787]: 	at System//com.google.common.collect.TransformedIterator.next(TransformedIterator.java:48)
[STDERR/]: [java.lang.Throwable$WrappedPrintStream:println:787]: 	at System//com.google.common.collect.Iterators$5.hasNext(Iterators.java:543)