squash: fix condition

Author: happz

tmt/guest/__init__.py

@@ -1989,7 +1989,8 @@ def plan_environment(self) -> Environment:
     def intrinsic_environment(self) -> Environment:
         environment = Environment()
 
-        environment['TMT_PLAN_ENVIRONMENT_FILE'] = EnvVarValue(self.plan_environment_path)
+        if self.plan_environment_path is not None:
+            environment['TMT_PLAN_ENVIRONMENT_FILE'] = EnvVarValue(self.plan_environment_path)
 
         return environment