traceback when using rez-plugins

```
Searching [ ] 22/1451Traceback (most recent call last):
File "/latest/payload/bin/rez/rez-plugins", line 8, in
sys.exit(run_rez_plugins())
File "/2.94.0/payload/lib/python2.7/site-packages/rez/cli/_entry_points.py", line 193, in run_rez_plugins
return run("plugins")
File "/2.94.0/payload/lib/python2.7/site-packages/rez/cli/_main.py", line 184, in run
returncode = run_cmd()
File "/2.94.0/payload/lib/python2.7/site-packages/rez/cli/_main.py", line 176, in run_cmd
return func(opts, opts.parser, extra_arg_groups)
File "/2.94.0/payload/lib/python2.7/site-packages/rez/cli/plugins.py", line 35, in command
pkgs_list = get_plugins(package_name=opts.PKG, paths=pkg_paths)
File "/2.94.0/payload/lib/python2.7/site-packages/rez/package_search.py", line 150, in get_plugins
if not plugin_pkg.plugin_for:
AttributeError: 'NoneType' object has no attribute 'plugin_for'
```

Start by reproducing the traceback with the rez-plugins command, then inspect rez/package_search.py at get_plugins and rez/cli/plugins.py where the command calls it. Done means the command no longer raises AttributeError when a plugin package lookup returns None, with a test covering the reported case.

Written by the indexing model from the issue text.