Ensure optional arrays only bind prefix if there are elements

Author: illusional

wdlgen/task.py

@@ -109,7 +109,8 @@ def get_string(self):
                         print("separate_array take preferences over: separator, default, true, false")
                     if self.optional:
                         # Ugly optional workaround: https://github.com/openwdl/wdl/issues/25#issuecomment-315424063
-                        internal_pref = f'if defined({name}) then "{bc}" else ""'
+                        # Additional workaround for 'length(select_first({name}, [])' as length requires a non-optional array
+                        internal_pref = f'if defined({name}) && length(select_first([{name}, []])) > 0 then "{bc}" else ""'
                         return f'~{{{internal_pref}}}~{{sep=" {bc}" {name}}}'
                     return f'~{{sep=" " prefix("{bc}", {name})}}'