Fix error message when reimporting resources with an empty scene open

[aaronfranke]

When reimporting, the import dock has code that will update the resource in all scenes currently opened in the editor.
If the scene is empty (like when no scene is open), this will fail and print this error message:

ERROR: Parameter "p_object" is null.
   at: _replace_resource_in_object (editor/import_dock.cpp:606)

This PR adds a check to see if the edited scene has a root node, and if not, don't try to update it.

This PR is very minor. The only behavior difference is the lack of a useless error message being printed.

[KoBeWi]

Looks good, but I'm wondering about likely. I don't remember seeing it used in a similar code.

[aaronfranke]

@KoBeWi likely instructs the compiler to tell the CPU to guess that this is going to be true most of the time. This is a micro-optimization and not really necessary, but I think it's slightly better.

[akien-mga]

We use unlikely in ERR_FAIL_COND checks, so if this has similar expectations for how rarely it would be untrue, likely seems OK.

[akien-mga]

Thanks!