Use be_control_look != NULL everywhere in the Interface Kit.

Should not be a functional change. It is not in the Haiku Coding Guidelines but
I feel like 'if (object != NULL)' is generally preferred to 'if (object)', plus
in this case of be_control_look that is the more common style.

src/kits/interface/ScrollBar.cpp

@@ -1081,7 +1081,7 @@ BScrollBar::Draw(BRect updateRect)
 		}
 
 		// fill the clickable surface of the thumb
-		if (be_control_look) {
+		if (be_control_look != NULL) {
 			be_control_look->DrawButtonBackground(this, rect, updateRect,
 				normal, 0, BControlLook::B_ALL_BORDERS, fOrientation);
 		} else {