Inconsistent behaviour with @MappingTarget, source objects and null values

### Expected behavior

I'd expect the targetvalues for the source object to be updated even if there is no dto present

### Actual behavior

The targetvalue for the source object will only be updated if the dto or at least one of the source objects is not null.
| DTO | source object| expected | actual |
|--------|--------|--------|--------|
| set | set | update with dto-contents and source object | update with dto-contents and source object |
| set | null | update with dto-contents and source object to null | update with dto-contents and source object to null |
| null | set | no update with dto-contents but updated source object |no update with dto-contents but updated source object |
| null | null | no update with dto-contents but updated source object to null | no update at all |

I think the issue is the guard clause at the beginning of the generated implementation. This should be ommitted if there's a parameter with a @MappingTarget annotation

### Steps to reproduce the problem

https://github.com/nicktar/mapstruct-mcve
The failing unit test demostrated the issue

### MapStruct Version

MapStruct 1.5.5-Final

Start with the failing unit test in the nicktar/mapstruct-mcve reproduction and inspect the generated implementation's guard clause. Verify the four DTO and source-object combinations, then confirm that a @MappingTarget is updated even when both the DTO and source object are null.

Written by the indexing model from the issue text.