Warnings on boot if gonio axes values are not set

If they are set then it works fine because the fetch returns a float.

```
Failed to get detector/api/1.8.0/config/phi_increment:
TypeError float() argument must be a string or a real number, not 'NoneType'
Failed to get detector/api/1.8.0/config/kappa_start:
TypeError float() argument must be a string or a real number, not 'NoneType'
Failed to get detector/api/1.8.0/config/phi_start:
TypeError float() argument must be a string or a real number, not 'NoneType'
Failed to get detector/api/1.8.0/config/chi_increment:
TypeError float() argument must be a string or a real number, not 'NoneType'
Failed to get detector/api/1.8.0/config/chi_start:
TypeError float() argument must be a string or a real number, not 'NoneType'
Failed to get detector/api/1.8.0/config/kappa_increment:
TypeError float() argument must be a string or a real number, not 'NoneType'
Failed to get detector/api/1.8.0/config/omega_start:
TypeError float() argument must be a string or a real number, not 'NoneType'
Failed to get detector/api/1.8.0/config/omega_increment:
TypeError float() argument must be a string or a real number, not 'NoneType'
Failed to get detector/api/1.8.0/config/two_theta_increment:
TypeError float() argument must be a string or a real number, not 'NoneType'
Failed to get detector/api/1.8.0/config/two_theta_start:
TypeError float() argument must be a string or a real number, not 'NoneType'
```

Check if this happens with a real detector. If the real detector returns None, then we need to handle it correctly. If it is just the simulator then that should be fixed.

## Steps To Reproduce

With sim
1. Start tickit simulator
2. Start IOC

or with real detector

1. Re-initialise the detector
2. Check what values these parameters have with curl
- e.g. `curl -v http://ixx-eiger01/detector/api/1.8.0/config/phi_start`
3. Start IOC

## Acceptance Criteria

- No warnings on boot