fix empty-string LEGEND_METADATA env var

[ManuelHu]

it is quite common to use ENV_VAR= command --args ... for this to unset an env var for a command

until now (in stable versions), pylegendmeta did try to use the current directory as a metadata repo.

• if ${CWD} was a git repo, it probably just failed later on, when something else does not find the right data.
• if ${CWD} was not a git repo, it fails right away with

  git.exc.GitCommandError: Cmd('git') failed due to: exit code(128)
    cmdline: git clone -v --recurse-submodules -- [email protected]:legend-exp/legend-metadata 
    stderr: 'fatal: could not create work tree dir '': No such file or directory
  

with the changes from #75. this might have bad consequences on unrelated git repos, as now legendmeta does try to check out tags in the git repo.

The solution implemented here treats an empty path as if None had been passed. The other option would be to throw an exception if an empty path is provided.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 57.81%. Comparing base (c248ade) to head (777d344).
Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
src/legendmeta/core.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #80       +/-   ##
===========================================
- Coverage   72.02%   57.81%   -14.21%     
===========================================
  Files           8        8               
  Lines         840      486      -354     
===========================================
- Hits          605      281      -324     
+ Misses        235      205       -30     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.