fix failing test in util.selector.format with imperial_length

the error is below, not sure how I haven't noticed it before 🤔

assert subject.format('imperial_length(1, 1)') == "1'"

>               raise VisitError(tree.data, tree, e)
E               lark.exceptions.VisitError: Error trying to process rule "imperial_length":
E
E               cannot access local variable 'output_unit' where it is not associated with a value

src/ifcopenshell-python/ifcopenshell/util/selector.py

@@ -233,7 +233,7 @@ class FormatTransformer(lark.Transformer):
 
     def imperial_length(self, args):
         if len(args) == 2:
-            input_unit = "foot"
+            input_unit, output_unit = "foot", "foot"
             value, precision = args
         else:
             value, precision, input_unit, output_unit = args