Error-prone X#searchForCause() implementation

The X#seachForCause utility method fails with a NullPointerException if the analyzed Throwable has no suppressed exception due to the missing null check.

I never encountered any issues with that with real usage of Ignite. However, writing a unit test that stabs the IgniteCache methods to throw an exception without initialized suppressed exceptions results in the stabbed method to return null instead of throwing an exception. The workaround is to properly initialize the suppressed list in the mocked exception.

I suggest to make the generic X#seachForCause utility method handle the case when the suppressed list is not initialized. That would simplify the unit tests code.