Fixing logic for biasType retrieval in the event there is no bias input

[LPanosTT]

The original method of retrieving the bias:

llvm::dyn_cast_or_null<::mlir::RankedTensorType>(getBias().getType());

resulted in a segmentation fault because getBias() itself would return an incomplete object. Thus, getBias().getType() would attempt to dereference a nullptr at some point.