Add error with zero length labels on GDS write.

compiler/gdsMill/gdsMill/vlsiLayout.py

@@ -422,7 +422,8 @@ class VlsiLayout:
         self.structures[self.rootStructureName].texts.append(textToAdd)
 
     def padText(self, text):
-        if(len(text)%2 == 1):
+        debug.check(len(text) > 0, "Cannot have zero length text string.")
+        if(len(text) % 2 == 1):
             return text + '\x00'
         else:
             return text
@@ -696,7 +697,6 @@ class VlsiLayout:
 
         return max_pins
 
-
     def getAllPinShapes(self, pin_name):
         """
         Search for a pin label and return ALL the enclosing rectangles on the same layer