`nikola build` fails with `base-jinja` theme and no `COMMENT_SYSTEM`

[clayadavis]

Environment

Python Version: 3.9

Nikola Version: master

Operating System: Pop!_OS 21.04

Description:

In the current master (67fe01f44), sites using base-jinja and derived themes can fail to build when no comment system is used. The nikola build command fails with the following exception:

jinja2.exceptions.UndefinedError: 'jinja2.environment.TemplateModule object' has no attribute 'comment_link_script'

Full traceback.

Git bisect suggests this bug was introduced with commit 251a3b2ad09e70a3701c089cee740f472fdccb95

Steps to reproduce

• Check out nikola@67fe01f44
• Build a demo site without comments, either leaving the commenting system blank in the interactive prompts or setting COMMENT_SYSTEM = "" in conf.py
• Set THEME = "base-jinja" in conf.py; child themes such as bootstrap4-jinja are also affected.
• Run nikola build