Fix for Issue #286: Creating a Federation Execution with a `null` or empty FOM array throws NullPointerException/IndexOutOfBoundsException

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Attempts to create a federation execution with a null for the FOM modules parameter failed with a NullPointerException, and with an empty FOM modules array caused an IndexOutOfBoundsException.

A few places in the code caused this, depending on which version/method signature of the createFederationExecution method is invoked, but they all basically came down to an assumption that the fomModules parameter is never a null or empty array. These are...

org.portico.lrc.services.federation.msg.CreateFederation:

• Added checks for a null URL (or URLs) in 3 constructor variants before processing to avoid NullPointerException

org.portico.impl.hla1516e.Rti1516eAmbassador#createFederationExecution( String federationName, URL[] fomModules, URL mimModule ):

• added check for null fomModules array before processing to avoid NullPointerException

org.portico.lrc.model.ModelMerger#mergeModels( List<ObjectModel> models ):

• added check for zero length List before processing to avoid IndexOutOfBoundsException

Two unit tests have also been added to cover the above situations and validate that behaviour is correct.

Tested with release build and all tests pass.