#236: Do not register problems on mapping composition annotations

[thunderhook]

closes #236

I tried to disable it by excluding it in the MapstructReferenceContributor when using the elementPattern` but somehow this did not get called.

Therefore I used the check inside shouldRegisterProblem(...) to exclude it.

[filiphr]

Thanks @thunderhook.

Does this also work properly if we have something like:

@Retention(RetentionPolicy.CLASS)
@Mappings({
    @Mapping(target = "id", ignore = true),
    @Mapping(target = "creationDate", expression = "java(new java.util.Date())"),
    @Mapping(target = "name", source = "groupName")
})
@interface ToEntity { }

[filiphr]

There is really no need to use ofNullable here. I prefer assigning this to a value and doing a good old regular null check.

[thunderhook]

Thanks @thunderhook.

Does this also work properly if we have something like:

@Retention(RetentionPolicy.CLASS)
@Mappings({
    @Mapping(target = "id", ignore = true),
    @Mapping(target = "creationDate", expression = "java(new java.util.Date())"),
    @Mapping(target = "name", source = "groupName")
})
@interface ToEntity { }

I had a look at it, and yes it works. Added a test for this as well.