dsdl_compiler: check for StopIteration exception

This fixes the exception happening with Python 3.7.

I'm assuming this has to do with:
https://www.python.org/dev/peps/pep-0479/

libuavcan/dsdl_compiler/libuavcan_dsdl_compiler/__init__.py

@@ -298,7 +298,10 @@ def make_template_expander(filename):
         def enum_last_value(iterable, start=0):
             it = iter(iterable)
             count = start
-            last = next(it)
+            try:
+                last = next(it)
+            except StopIteration:
+                return
             for val in it:
                 yield count, False, last
                 last = val