Resolve undefined behavior with VLAs length

```
RecoEcal/EgammaClusterAlgos/src/Multi5x5BremRecoveryClusterAlgo.cc:47:34:
runtime error: variable length array bound evaluates to non-positive
value 0
RecoEcal/EgammaClusterAlgos/src/Multi5x5BremRecoveryClusterAlgo.cc:50:30:
runtime error: variable length array bound evaluates to non-positive
value 0
```
`Multi5x5BremRecoveryClusterAlgo::makeIslandSuperClusters(..)` is called
with empty `clusters_v`. There is nothing to be done if `clusters_v` is
empty, thus bail out early.

This happens 36 times while running short matrix.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>