Possible enhancement needed to request file license auto-generation

### Low priority issue:
When working on[ this issue]( https://github.com/MetOffice/CDDS/pull/869), i tried removing the `license` field (both key and value/license string)for a cmip6plus request file. https://github.com/MetOffice/CDDS/pull/869

When running prepare_generate, the current functionality worked for generating the correct license field with its appropriate value in the output `mip_convert` files.

However, i found if you remove the string in the request file for the license e.g.

```
license =
```

Then run `prepare_generate`. The `mip_convert` files generated will not generate the appropriate license string.

There may be some value to it working this way if someone can think of one? However, it would probably be good to always add the license if the value for the `licence` field has been left empty.

Start by tracing the prepare_generate entry point and how it reads the license field from CMIP6Plus request files. Compare generation with the field omitted versus `license =`; done means the generated mip_convert files contain the appropriate license string in both cases, with tests or equivalent verification for the empty-value case.

Written by the indexing model from the issue text.