[1.3.0] blurb crashes both with absent or empty GitHub issue (and should help with how to appraoch that case) #34

# blurb  add
Traceback (most recent call last):
  File "[..]/cpython/venv/bin/blurb", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "[..]/cpython/venv/lib/python3.11/site-packages/blurb/blurb.py", line 1273, in main
    sys.exit(fn(*filtered_args, **kwargs))
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "[..]/cpython/venv/lib/python3.11/site-packages/blurb/blurb.py", line 940, in add
    blurb.load(tmp_path)
  File "[..]/cpython/venv/lib/python3.11/site-packages/blurb/blurb.py", line 540, in load
    self.parse(text, metadata=metadata, filename=filename)
  File "[..]/cpython/venv/lib/python3.11/site-packages/blurb/blurb.py", line 530, in parse
    finish_entry()
  File "[..]/cpython/venv/lib/python3.11/site-packages/blurb/blurb.py", line 485, in finish_entry
    if key == "gh-issue" and int(value) < lowest_possible_gh_issue_number:
                             ^^^^^^^^^^
ValueError: invalid literal for int() with base 10: ''

# blurb add
Traceback (most recent call last):
  File "[..]/cpython/venv/bin/blurb", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "[..]/cpython/venv/lib/python3.11/site-packages/blurb/blurb.py", line 1273, in main
    sys.exit(fn(*filtered_args, **kwargs))
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "[..]/cpython/venv/lib/python3.11/site-packages/blurb/blurb.py", line 962, in add
    path = blurb.save_next()
           ^^^^^^^^^^^^^^^^^
  File "[..]/cpython/venv/lib/python3.11/site-packages/blurb/blurb.py", line 637, in save_next
    filename = blurb._extract_next_filename()
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "[..]/cpython/venv/lib/python3.11/site-packages/blurb/blurb.py", line 628, in _extract_next_filename
    return path
           ^^^^
UnboundLocalError: cannot access local variable 'path' where it is not associated with a value