[5.3] Ignore return from closure in has one with default feature.

[sileence]

In order to prevent a potential issues / inconsistencies with closures not returning a valid model, let's use the closure as a way to customize the default instance but ignoring the returned value.

In order words, passing a closure or an array to withDefault will be a way to customize the default instance not to replace it.

Fix: #16614

[taylorotwell]

That is less flexible.

[patrickcarlohickman]

@taylorotwell Unfortunately, it seems it is a little too flexible. Would you mind making a comment on #16614? Sorry for the wall of text, but the gist is that if a HasOne relationship returns something other than null or a Model instance, it can break code (e.g. the push() method).