classes: bundle: prevent from splitting None
classes: bundle: prevent from splitting None

Otherwise, an empty 'RAUC_BUNDLE_SLOTS' variable will trigger a python
exception:

| Exception: AttributeError: 'NoneType' object has no attribute 'split'

Signed-off-by: Enrico Jorns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

rauc/meta-rauc,rauc/meta-rauc,rauc/meta-rauc,rauc/meta-rauc