Don't try to call .lower() on None

@emord 
I found this while testing, then checked my inbox for "has no attribute lower" and saw this looks like an actual problem.

```
  File "submodules/dimagi-utils-src/dimagi/utils/excel.py", line 146, in set_field_value
    }[value.lower()]

AttributeError: 'NoneType' object has no attribute 'lower'
```