simplify

Author: willmcgugan

src/textual/layout.py

@@ -256,11 +256,11 @@ def get_content_height(
         Returns:
             Content height (in lines).
         """
-        if not widget._nodes:
-            height = 0
-        else:
+        if widget._nodes:
             arrangement = widget._arrange(Size(width, 0))
             height = arrangement.total_region.bottom
+        else:
+            height = 0
 
         return height