CoreSetMIPSampling class hangs when lSet is empty

[Fsssshuo]

Issue Description: When using the query method of the CoreSetMIPSampling class, if the lSet parameter is empty, the program enters an infinite wait state and hangs. This occurs during the call to the getrepresentation method, which expects a non-empty set of indices.

Suggested Fix: Before calling the getrepresentation method, a check should be added to determine if lSet is empty. If it is, an alternative index set (e.g., uSet) should be used to obtain the representation.