Fix attribute fetching, which is needed to calculate mining waste of crystal'd strip miners

This potentially can break many things. Some of those are fixed already, but there might be many more...

gui/builtinViewColumns/attributeDisplay.py

@@ -80,7 +80,7 @@ class AttributeDisplay(ViewColumn):
             else:
                 attr = mod.getAttribute(self.info.name)
 
-        if attr is None:
+        if not attr:
             return ""
 
         if self.info.name == "volume":