Fix script class icons not inheriting parent

[Z0rb14n]

Closes #102565 by re-adding an omitted check for an empty icon path. While doing git bisect, identified this commit as the cause of the problem: a20934c (see removed check). There was a check for the return value being empty that was accidentally omitted, so this PR adds it back.

Sample I used for testing:

@icon("res://icon.svg")
extends Node2D
class_name BaseClass

extends BaseClass
class_name ChildClass

Previously, ChildScene icon would be a circle as per the issue.

[akien-mga]

Thanks!