trestle author profile-assemble `IndexError: string index out of range` while processing markdown

[srmamit]

Describe the bug

Running trestle author profile-assemble causes IndexError. This issue came up after the update to version 2.1.0. The same command exits with code 0 on trestle version 2.0.0

trestle author profile-assemble -v \
      --markdown "md_dir/profile" \
      --output profile \
      --set-parameters

Error Log:

trestle.core.commands.author.profile:96 ERROR: Assembly of markdown to profile failed: string index out of range
Traceback (most recent call last):
  File "~/venv/lib/python3.10/site-packages/trestle/core/commands/author/profile.py", line 190, in _run
    return self.assemble_profile(
  File "~/venv/lib/python3.10/site-packages/trestle/core/commands/author/profile.py", line 349, in assemble_profile
    found_alters, param_dict, param_map = catalog_api.read_additional_content_from_md(label_as_key=True)
  File "~/venv/lib/python3.10/site-packages/trestle/core/catalog/catalog_api.py", line 106, in read_additional_content_from_md
    return self._reader.read_additional_content(
  File "~/venv/lib/python3.10/site-packages/trestle/core/catalog/catalog_reader.py", line 65, in read_additional_content
    sort_id, control_alters, control_param_dict = ControlReader.read_editable_content(
  File "~/venv/lib/python3.10/site-packages/trestle/core/control_reader.py", line 359, in read_editable_content
    yaml_header, control_tree = md_api.processor.process_control_markdown(control_path, cli_section_dict, part_label_to_id_map)  # noqa: E501
  File "~/venv/lib/python3.10/site-packages/trestle/core/markdown/markdown_processor.py", line 86, in process_control_markdown
    tree = ControlMarkdownNode.build_tree_from_markdown(lines)
  File "~/venv/lib/python3.10/site-packages/trestle/core/markdown/base_markdown_node.py", line 61, in build_tree_from_markdown
    ob, _ = ob._build_tree(lines, 'root', 0, start_level)
  File "~/venv/lib/python3.10/site-packages/trestle/core/markdown/control_markdown_node.py", line 188, in _build_tree
    subtree, i = self._build_tree(lines, line, i + 1, level + 1, part_id)
  File "~/venv/lib/python3.10/site-packages/trestle/core/markdown/control_markdown_node.py", line 188, in _build_tree
    subtree, i = self._build_tree(lines, line, i + 1, level + 1, part_id)
  File "~/venv/lib/python3.10/site-packages/trestle/core/markdown/control_markdown_node.py", line 188, in _build_tree
    subtree, i = self._build_tree(lines, line, i + 1, level + 1, part_id)
  [Previous line repeated 1 more time]
  File "~/venv/lib/python3.10/site-packages/trestle/core/markdown/control_markdown_node.py", line 218, in _build_tree
    i = self._process_part_line(i, line, lines, content.part, read_parts=read_parts)
  File "~/venv/lib/python3.10/site-packages/trestle/core/markdown/control_markdown_node.py", line 427, in _process_part_line
    if line.lstrip()[0] != '-' or not read_parts:
IndexError: string index out of range

To Reproduce

Steps to reproduce the behavior:

1. Create a profile markdown file with content resembling following or insert the below line in a section of an existing valid profile:

This line is present before the potential tab or spaces in line below:
	
Following line to the preceding line

2. Run trestle author profile-assemble after the edit.

Expected behavior

profile.json assembled from markdown and command exit with code 0.

Environment

• OS: macos
• Python version: 3.8.13
• Installed packages: compliance-trestle==2.1.0