Deal with none case

Author: stxue1

cwltool/command_line_tool.py

@@ -889,7 +889,7 @@ def remove_prefix(s: str, prefix: str) -> str:
 
             # If there are environmental variables to preserve, add it to the key
             env_var_requirement = cast(dict[str, str], keydict.get("EnvVarRequirement", {}))
-            env_def = cast(CWLObjectType, self.get_requirement("EnvVarRequirement")[0])
+            env_def = cast(CWLObjectType, self.get_requirement("EnvVarRequirement")[0] or {})
             if runtimeContext.preserve_environment is not None:
                 env_def.update(JobBase.extract_environment(runtimeContext, env_var_requirement))