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

[hartwork]

This is blurb 1.3.0 with line .. gh-issue:…

# 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: ''

…and this is blurb 1.3.0 with line # .. gh-issue: (i.e. commented out):

# 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

Neither case should crash and the text would ideally help about making that case work where adequate.

Thanks in advance!

[hugovk]

Please see PR #35 to fix the first part.

[hartwork]

@hugovk cool!

[hugovk]

And updated to include the second part as well.