panic: cast causes pointer to be null when creating an OwningGroup without entities

# Steps to reproduce

Run the view_vs_groups.zig example but comment the `createEntities(&reg)` function
https://raw.githubusercontent.com/prime31/zig-ecs/master/examples/view_vs_group.zig

# Expected

Program to run as normal. There won't be anything to iterate, but that shouldn't crash the program.

# Actual

```panic: cast causes pointer to be null```