small fix for tool.Drawing.get_drawing_metadata

Otherwise it always returned a list with 1 empty string like `[ "" ]` which resulted in messy class names in svg like "-87IfcAnnotation0LtaLiEpzAOxEqmIvCV27EFALLSLOPEANGLE".

src/blenderbim/blenderbim/tool/drawing.py

@@ -960,6 +960,7 @@ class Drawing(blenderbim.core.tool.Drawing):
         return [
             v.strip()
             for v in ifcopenshell.util.element.get_psets(drawing)["EPset_Drawing"].get("Metadata", "").split(",")
+            if v
         ]
 
     @classmethod