Avoid using 'slightly' since this could be a considerable performance improvement depending on the provider.
Co-authored-by: Matthias Kuhn <matthias@opengis.ch>
Up to date it was not possible to create a function that handles NULL values with the
@qgsfunction decorator. As soon as any parameter was NULL, the return value would also
be NULL.
Example of a function that returns a value now with a NULL paramter and would have returned NULL before
```
@qgsfunction(args=-1, group='Custom', handlesnull=True)
def mean_value(vals, feature, parent):
valid_vals = [val for val in vals if val != NULL]
return sum(valid_vals)/len(valid_vals)
```
[FEATURE]
